BLS WORKING PAPERS 

 U.S. Department of Labor 

U.S. Bureau of Labor Statistics

Office of Employment and Unemployment Statistics 

Tobit or Not Tobit?

 

Jay Stewart, U.S. Bureau of Labor Statistics

Working Paper 432

November 2009

All views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the

U.S. Bureau of Labor Statistics.
Tobit or Not Tobit? 

October 2009 

Jay Stewart* 

U.S. Bureau of Labor Statistics 

2 Massachusetts Ave., NE 

Washington, DC 20212 

USA 

Stewart.Jay@bls.gov

(202) 691-7376 

* Any views expressed here are mine and do not necessarily reflect those of the Bureau of Labor 

Statistics. I thank Rachel Connelly, Lewis Davis, Matt Dey, Tim Erickson, Harley Frazis, Dan 

Hamermesh, Sabrina Pabilonia, and participants of seminars at the Bureau of Labor Statistics 

and Union College for their helpful comments and suggestions. 
Tobit or Not Tobit? 

Abstract 

Time-use surveys collect very detailed information about individuals’ activities over a short 

period of time, typically one day. As a result, a large fraction of observations have values of 

zero for the time spent in many activities, even for individuals who do the activity on a regular 

basis. For example, it is safe to assume that all parents do at least some childcare, but a 

relatively large fraction report no time spent in childcare on their diary day. Because of the large 

number of zeros Tobit would seem to be the natural approach. However, it is important to 

recognize that the zeros in time-use data arise from a mismatch between the reference period of 

the data (the diary day) and the period of interest, which is typically much longer. Thus it is not 

clear that Tobit is appropriate. 

 In this study, I examine the bias associated with alternative estimation procedures for 

estimating the marginal effects of covariates on time use. I begin by adapting the infrequency of 

purchase model, which is typically used to analyze expenditures, to time-diary data and showing 

that OLS estimates are unbiased. Next, using simulated data, I examine the bias associated with 

three procedures that are commonly used to analyze time-diary data—Tobit, the Cragg (1971) 

two-part model, and OLS—under a number of alternative assumptions about the data-generating 

process. I find that the estimated marginal effects from Tobits are biased and that the extent of 

the bias varies with the fraction of zero-value observations. The two-part model performs 

significantly better, but generates biased estimated in certain circumstances. Only OLS 

generates unbiased estimates in all of the simulations considered here. 

Jay Stewart 

U.S. Bureau of Labor Statistics 

2 Massachusetts Ave., NE 

Washington, DC 20212 

USA 

Stewart.Jay@bls.gov

(202) 691-7376 
Introduction 

 With the introduction of the American Time Use Survey (ATUS), there has been a 

renewed interest in research using time-diary data. One feature of these data is that a large 

fraction of individuals have zero values for the time spent in many activities. So far there has 

been no general agreement on the correct approach to dealing with these zero-value observations. 

Researchers have used OLS (Bonke, 1992; and Frazis and Stewart, forthcoming), a two-part 

model similar to the one proposed by Cragg (1971) (Cawley and Liu, 2007), and Tobit (SouzaPoza, Schmid, and Widmer, 2001; Kalenkoski, Ribar, and Stratton, 2005; Kimmel and Connelly, 

2007). Some authors report estimates from more than on estimation procedure (Hamermesh, 

2009; Price, 2008). However, Tobit has been the predominant approach in more-recent studies. 

The Tobit model would seem to be a sensible approach, because it was developed specifically for 

situations where the dependent variable is truncated at zero or some other cutoff. 

The standard discussion of the Tobit model (Tobin, 1958) assumes that there is a latent 

variable (for example, desired expenditures) underlying the observed dependent variable (actual 

expenditures). The two are equal when the latent variable is greater than zero, but the observed 

variable is zero when the latent variable is negative. In economic models, this corresponds to a 

corner solution in the utility maximization program where the individual’s optimal value of the 

dependent variable is negative but nonnegativity constraints force the value to be zero. It is wellknown that, under these assumptions, OLS parameter estimates are downward biased and 

inconsistent while Tobit estimates are consistent and asymptotically normal (Amemiya, 1973).1

 

This interpretation is not generally appropriate for time-diary data, because a zero does 

not necessarily imply that the individual never does the activity. The fixed costs of engaging in 

 

1

 This assumes that the distribution of errors is normally distributed. 
- 2 -

an activity may lead the individual to do the activity on some days but not on others, as can 

random events such as illness or a change of schedule.2

 Important examples of activities that are 

not done every day include time spent working by the employed, time spent looking for work by 

the unemployed, and time spent in childcare by parents. In these cases, zeros arise because the 

reference period of the data (the diary day) is shorter than the period of interest (i.e., the period 

over which decisions are made). In this sense, time-diary data are very similar to expenditure 

data. For example, expenditures on clothing are often zero in expenditure diaries, but casual 

empiricism suggests that virtually everybody purchases clothing. 

The infrequency of purchase model (IPM) was specifically developed to address the 

mismatch between the period of interest and the reference period of the data. It has been shown 

that using OLS to estimate the effect of income on consumption in an IPM framework results in 

biased and inconsistent parameter estimates (Keen, 1986). Alternatives to OLS include an 

instrumental variables estimator proposed by Keen (1986) and a two-part model along the lines 

of Cragg (1971), which generalizes the Tobit model (Blundell and Meghir, 1987).3

 

 Along these lines, there is an alternative interpretation of the Tobit model that does not 

assume there is a latent variable that takes on negative values. Instead, it only assumes that there 

is information conveyed in the probability that the dependent variable is equal to zero (see 

McDonald and Moffitt 1980). But even if this interpretation is correct Tobit still may not be 

appropriate because, as Cragg (1971) points out in the context of estimating expenditure models, 

 

2

 The standard household production model can be modified to incorporate timing (for example, see Stewart, 

forthcoming). In this type of model, it can easily be shown that if the daily fixed cost of engaging in an activity is 

sufficiently high, the individual will not engage in the activity every day. 

3

 Cragg (1971) proposes a double-hurdle model, where the first hurdle is the decision to ever spend money on the 

good. Since I am restricting my attention to situations where this decision is taken as given, the double-hurdle 

model reduces to a two-part model. In the first part of the two-part model, a probit is estimated over all observations 

to determine the probability that individuals purchase the good during the reference period. In the second part, an 

OLS regression is estimated over the non-zero-value observations. The estimated average probability from the 

probit is combined with the coefficients from the OLS regression to arrive at unconditional marginal effects. 
- 3 -

it assumes that the process that determines whether a person purchases a good is the same as the 

one that determines the amount spent on that good. 

In the analysis that follows, I examine the appropriateness of alternative procedures for 

estimating the effects of covariates on the average amount of time spent in an activity when there 

are zero-value observations in the data. I begin by adapting the IPM to time-diary data and 

showing that, in this context, OLS is unbiased. Next, I generate simulated time-use data using 

the IPM framework and estimate the effect of covariates on time use using OLS, Tobit, and the 

two-part model. I compute the bias associated with each procedure, and examine how the bias 

and mean squared error (MSE) vary with the fraction of zero-value observations in the data. 

Adapting the Infrequency of Purchase Model to Time-Diary Data 

Using the notation from Keen (1986), expenditures on good k are equal to: 

(1)  

hk

hkhkhk

hk

p

ucw

e

  , k = 1,...,N, 

where ehk and hk c denote expenditures and consumption of good k by household h, phk denotes 

the probability that good k is purchased during the reference period, whk is an indicator that 

equals 1 if household h is observed purchasing good k during the reference period, and uhk is a 

random term (where E(uhk ) = 0) that captures variation in the amount spent on good k. Note that 

uhk is constrained such that uhk ≥ – hk c (so that expenditures are always non-negative), and the 

two random terms whk and uhk are assumed to be independently distributed. 

The terms in equation (1) have natural interpretations in the context of time-diary data. 

Individuals determine how much time they wish to spend in each activity over some period of 

time, such as a month, and then allocate that time to individual days. Using the notation of the 

IPM, hk c is the amount of time that the individual spends in activity k each month (expressed as 
- 4 -

a daily average) and ehk is the observed amount of time spent doing activity k on the diary day. 

The remaining terms in equation (1) have analogous interpretations—phk is the probability that 

the individual does activity k on any given diary day, whk = 1 if the individual engaged in activity 

k on the diary day, and uhk is a random term that captures day-to-day variation in the amount of 

time spent in activity k. If hk c is a linear function of a set of covariates (to keep things simple, I 

consider the one-covariate case), then: 

(2) hkkhk c    x , 

where xh is a covariate that is thought to influence time spent in activity k. Combining equations 

(1) and (2) yields: 

(3) ,

){( /}

hkhkk

hkkhk hkhkhkhkhkhk

x

e puwcpwx



 



  

which can be estimated using OLS. 

 Expressing ehk, hk c , and xhk as deviations from their respective means (and using the 

“dot” notation), the estimated coefficient,  k

ˆ , is given by: 







   

h h

h hkhkh

h h

h hkh

k

x

xx

x

ex

2 2

)( ˆ



 



   , 

where hk hkhkhkhkhkhk  ){(   /} puwcpw . Arranging terms and taking expectations, we have: 













































h hk h

h hkhk

h hk h

h hkhkh

kk

xp

uw

E

xp

cwx

E E 2 2 ) ˆ (  

   , 

where ) ( hk  pww hkhk  . Substituting equation (2) for hk c

 yields: 

) . ˆ ( 2 2

2













































h hk h

h hkhk

h hk h

h hkhk

kk

xp

uw

E

xp

wx

E E  

  
- 5 -

Given the assumption that E(uhkwhk) = E(uhk) = 0, the third term is equal to zero. The second 

term is also equal to zero as long as 0)( 2

wxE  hkh  . Because whk  is the deviation of whk around 

its mean value of phk, 0)( 2

wxE  hkh  even if phk is a function of xh. Thus, E   kk ) ˆ ( and 

estimating equation (3) using OLS will generate unbiased estimates of βk . 

 The rest of the paper is devoted to comparing three alternative estimation procedures that 

have been used with time-diary data: OLS, Tobit, and Cragg’s two-part model. I construct a 

simulated sample using the data-generating process described above, and then use these three 

models to estimate parameters under alternative assumptions about the fraction of zero-value 

observations. 

Construction of the Simulated Data 

To construct the sample for the simulations, I started by assuming that all individuals are 

“doers” (i.e., that they do activity k for at least a few minutes every month). To allow for 

random variation (due to unobserved factors) in the amount of time spent in activity k and to 

make the simulation more consistent with the assumptions of the Tobit model, I modified the 

adapted IPM slightly by adding a normally-distributed error term, θhk, to equation (2). For 

example, if activity k is childcare then θhk might be large and positive for the month if a child 

stayed home from school for a few days with the flu and required additional care. Thus equation 

(2) becomes: 

(2′) hk Xc   hkkh , 

where Xh is a vector of covariates (including an intercept), Βk is a vector of coefficients, and the 

error ),0(~ 2

hk hk N   and is uncorrelated with whk or uhk. Assuming three covariates and 

dropping activity subscripts to reduce clutter, equation (2′) becomes: 
- 6 -

(2′′) h h h hh        xxxc 332211  , 

where α = 10, β1 = 1.5, β2 = 3, and β3 = 2. The data for the xi and θh were generated using a 

random number generator and are distributed as follows: x1 ~ U[1,4], x2 ~U[2,3], x3 is Bernoulli 

with Prob(x3 = 1) = 0.5, and θh ~ N(0,1).4

 The assumption that α = 10 ensures that h c > 0 for 

nearly all respondents. The sample size for each simulation was 50,000 observations, minus the 

small number of “respondents” who were dropped because h c ≤ 0. 

The next step was to generate daily values for time spent in activity k. For each 

individual in the sample, I created 28 days of data and generated the amount of time spent in 

activity k each day. Combining equations (1) and (2′), the amount of time spent doing activity k

on day d is given by: 

(4) 

h

hdhd

h

hhd

h

hhd

h

hdhhd

hd

p

uw

p

w

p

Xw

p

ucw

e 

    )(  . 

Taking expectations verifies that: 

( . hd ) XeE h

I implemented equation (4) as follows. First, I set ehd = h c × δ, where δ ~ U[0,1] on 

weekdays and δ ~ U[0,2] on weekends. Note that this implies individuals spend more time in 

activity k on weekends and that, by construction, ehd > 0 for all days. To generate zero 

observations, I sorted the days for each individual by ehd, and set ehd = 0 for the Th days with the 

lowest values for ehd. Thus, ph = Th/28. The values of ehd for the remaining (28 − Th) days are 

inflated proportionately so that  

d hd h 28 ce . Note that this last step also preserves the 

normality of θh. 

 

4

 The results are not sensitive to the variance of θh. I ran several sets of simulations with θh ~ N(0,2), and got nearly 

identical results. 
- 7 -

I ran seven sets of simulations, each of which used a different algorithm to determine the 

relationship between Th and the variables in the model (see the Appendix for a description of the 

algorithms). All of the algorithms have a random component so that the fraction of zero 

observations varies across individuals. Noting that E(wh) = ph and letting Nww

h   hd , where 

N is the final sample size, the fraction of zero observations is ݍത = (1–w ). The relationships 

between qh (= 1 – ph) and the variables in the model are: 

(1) qh is unrelated to the value of h c or any of the xi. 

(2) qh is negatively related to the value of h c . 

(3) qh is negatively related to the value of x1. 

(4) qh is positively related to x2. 

(5) qh is negatively related to the value of x3. 

The first set of simulations, while not very realistic, provides a useful baseline. I present three 

sets of simulations for (2), because this would seem to be the most likely case. Cases (3) – (5) 

cover situations where one of the covariates affects qh directly rather than indirectly through 

their effects on h c . For each xi, I ran two sets of simulations—one where qh is a positive 

function of xi and one where the relationship is negative—but I only report the simulations that 

resulted in a negative correlation between qh and h c .

5

 For each set of simulations, I varied the 

values of the Th so that the fraction of zero-value observations ranged between 0 and 0.9.6

 

 

5

 The other simulation results are available from the author on request. 

6

 To estimate the two-part model, it was necessary to truncate the range to between 0.005 and 0.9. 
- 8 -

Simulation Results 

Once the data were generated, I randomly selected one day for each individual in the 

sample, and estimated the βs using OLS, Tobit, and the two-part model. I report the estimated 

coefficients from OLS and unconditional marginal effects for the Tobit and two-part models.7

 

For OLS and Tobit, I simply estimated the simulation version of equation (3) over all 

observations in the sample: 

(5) hd h h hdh e xxx 332211   . 

The unconditional marginal effects for the Tobit model were computed as: 

 

















 





  

T

j hj

T

j

T

T

i

hi

hd

x

x

eE



  ˆ

ˆ ˆ ˆ )|(

3

x 1

using the mfx command in STATA,8

 where the T superscript indicates the Tobit coefficients. 

For the two-part model, I estimated: 

 

hd h h hdh w     xxx   3322110 over all observations using probit, and 

 P

hdh

P

h

P

h

PP

hd e xxx

2

3

2

32

2

21

2

1

2   over observations for which ehd > 0 using OLS. 

The marginal effects were computed as: 

 

  . ˆ

ˆ

ˆˆ ˆ ˆ

ˆ

ˆˆ

)|( 2

3

1 3 0

1 0|

2 2

3

1 0

P

i

j hjj

j ehj

PT

j

P

hi

j hjj

hi

hd

x

x

x

x

x

eE

hd 





 

























 

 





















 



 

  





 



x

Figures 1-5 show the simulation results. The three panels in each figure correspond to 

the three procedures, and show the bias in the estimated marginal effects for the covariates, 

 

7

 I also examined the bias associated with using Tobit coefficients, rather than marginal effects. The coefficients 

generally overestimated the true parameters, with the bias increasing sharply as ݍത. increases. I do not report the 

coefficients, because they are rarely reported in time-use research. 8

 Note that for x3, the Bernoulli-distributed covariate, I used the STATA default of computing the marginal effect as 

the effect of a discrete jump between 0 and 1. 
- 9 -

expressed as a percentage of the true parameter values and graphed against q , the fraction of 

zero observations. I computed the bias as ሺܧܯ ௜ ෢െ ߚ௜ሻ/ߚ ௜so that a positive value indicates that 

the magnitude of βi has been overestimated, while a negative value indicates that the magnitude 

has been underestimated. A wrong-signed coefficient would cause the bias to be less than −1. 

Figure 1 shows the baseline set of simulations, where qh is independent of any of the 

variables in the model. The Tobit marginal effects underestimate the true effects, and the 

magnitude of this bias increases with q . The bias is large (about 30 percent) when q = 0.4, and 

grows to over 50 percent when q > 0.8. In contrast to the Tobit model, both the OLS and the 

two-part model generate estimates that are unbiased, even as q becomes large. What does 

happen is that as q increases (greater than about 0.7), the variability of these estimates becomes 

quite large. For x1 and x3 the parameter estimates are off by over 30 percent in a few cases, while 

for x2 the parameter estimates are never off by more than 10 percent. Even so, these extreme 

estimates still have smaller bias than the average Tobit estimates. 

Table 1 shows the MSE of the estimated coefficients and marginal effects for different 

ranges of q , where each panel corresponds to a different figure. We can see that, for all three 

procedures, the MSE increases as q increases. Despite the considerable variability in the OLS 

and two-part model estimates, the small bias in both procedures results in MSEs that are less 

than 5 percent of the Tobit estimates’ MSE. 

As noted above, it is more realistic to assume that individuals who spend more time per 

month doing an activity are less likely to report zero time spent in the activity on their diary day. 

The simulation results in Figures 2a, 2b, and 2c show cases where qh is negatively related to h c . 

The results in the three figures are fairly similar to those in Figure 1. As in Figure 1, OLS 
- 10 -

generates unbiased estimates, with the variability of these estimates increasing as q increases, 

while Tobit marginal effects underestimate true parameter values. There is a slight bias in the 

two-part model (less than 5 percent) over some values of q . However, the main differences 

between these figures and Figure 1 are that the Tobit marginal effects are closer to the true 

values, and that the magnitude of the bias does not increase as rapidly with q . In Figure 2a, the 

bias is smaller than in Figure 1, but is still quite large. In Figures 2b and 2c, the marginal effects 

estimates are mostly within about 10 percent of the true parameter values for values of q < 0.7, 

and are fairly close to the lower bounds of the estimates from OLS and the two-part model.9

 

The MSEs that correspond to Figures 2a, 2b, and 2c, exhibit the same pattern as those 

corresponding to Figure 1, with MSEs increasing as q increases and Tobit marginal effects 

having larger MSEs. But in these simulations, the difference between Tobit and the other two 

procedures has narrowed. For larger values of q Tobit MSEs are smaller than those 

corresponding to Figure 1, while MSEs for OLS and the two-part model are about the same as in 

Figure 1 or slightly larger. However, Tobit MSEs are still considerably larger than those of the 

other two procedures. 

The performance of Tobit and the two-part model deteriorates in Figures 3-5, where qh is 

a direct function of one of the covariates. Tobit marginal effects are still downward biased, 

except for the marginal effects on the covariate that directly affects qh . For example, the bias in 

the marginal effect of x1 is positive and increases rapidly with q when qh is a positive function 

 

9

 I also ran a set of simulations where h q was positively related to chk. Both OLS and the two-part model generated 

unbiased estimates until ݍത reached the 0.75 to 0.80 range, at which point the estimates became downward biased for 

all three coefficients. Tobit marginal effects were downward biased, with the bias being quite large. These results 

are available from the author on request. 
- 11 -

of x1.

10 The two-part model performs better than Tobit, but some coefficients are biased, with 

the pattern of bias depending on which covariate directly affects qh . In Figure 3, where qh is a 

function of x1, we see that the marginal effect of x1 is unbiased for q < 0.6. For larger values of 

q , the bias in the marginal effect is positive and increasing, while the bias in the marginal effects 

of x2 and x3 are negative and decreasing. This pattern is similar to what was observed for Tobit, 

but less extreme. In Figure 4, the bias does not appear to be particularly severe. But in Figure 5, 

where qh is a function of x3, the marginal effect of x3 is downward biased for q > 0.4, although 

the marginal effects of the other two covariates are still unbiased for all values of q . 

In contrast to the Tobit and two-part models, nothing changes when qh is a direct function 

of one of the covariates. OLS coefficients are still unbiased, and the variation of these 

coefficients still increases as ݍത becomes large. 

Turning back to the MSEs in Table 1, we see that OLS and the two-part model are fairly 

close in most cases. In Figure 3, the MSEs for OLS and the two-part model are nearly identical 

for all three covariates until q > 0.6, with the differences becoming quite large for ݍത > 0.8. In 

Figure 5, the only difference is for x3. Contrary to the other sets of simulations, the MSE for x3 is 

largest when ݍത is in the 0.6 - 0.8 range, and then becomes smaller as ݍത approaches 0.9. 

Discussion and Conclusions 

The simulation results clearly show that marginal effects from the Tobit model are 

biased, that the bias is often large, and that the extent of the bias increases as the fraction of zero 

observations increases. It seems likely that one of the main reasons for this poor performance is 

 

10 In the opposite case, where the resulting correlation between the covariates and ݍത results in a positive correlation 

between ݍത and the time spent in the activity, all three coefficients are downward biased with the bias increasing as ݍത

increases. 
- 12 -

that the Tobit model assumes that the process that determines whether an individual engages in 

an activity is the same one that governs how much time is spent in that activity. This explanation 

is consistent with the findings of Daunfeldt and Hellstrom (2001) who, in their study of time 

spent in household production activities, reject the Tobit model in favor of the two-part model. 

My simulations confirm that the two-part model performs better than Tobit. As long as the 

probability of doing the activity on a given day does not depend on any of the covariates, the 

two-part model generates estimated marginal effects that are unbiased and invariant to the 

fraction of zeros in the data. However, if the probability of doing the activity on any given day is 

a function of one of the covariates, the two-part model behaves unpredictably. This is 

unfortunate, because a potential advantage of the two-part model is the ability to decompose the 

marginal effects to examine the effects of covariates on incidence and intensity. 

In contrast to the two models that were specifically designed to address the problem of 

zero observations, OLS estimates are unbiased and robust to a number of assumptions about the 

relationship between the variables in the model and the probability of doing the activity. Both 

OLS and the two-part model outperform Tobit in all simulations. There is virtually no difference 

between OLS and the two-part model, except in cases where the probability of doing the activity 

is a function of one of the covariates—in these cases, OLS outperforms the two-part model. 

There are two issues that I did not address in this study: standard errors and what happens 

if it is not possible to identify doers. The presence of zeros in the data are likely to affect 

standard errors by introducing heteroskedasticity into the residual. However, using robust 

methods to compute standard errors should address this problem. 

The second issue is more serious. If it is not possible to identify doers, then none of the 

three procedures performs particularly well. I ran two sets of simulations where a fraction of the 
- 13 -

sample included non-doers as well as doers. The simulated data were constructed as described 

earlier, except that the intercept was adjusted downward to generate more zeros.11 This is 

essentially the Tobit assumption. As in the simulations described above, the Tobit model 

generates downward biased estimates and the bias increases as the fraction of zero observations 

(including non-doers) increases. The estimated marginal effects from the two-part model have a 

large bias when the total fraction of zero observations is only slightly larger than the fraction of 

“true” zero observations, and the bias decreases as the fraction of zero observations increases (as 

true zeros become a smaller fraction of all zero observations). It appears that it is the mixture of 

true zeros and reference-period-mismatch zeros that leads to biased estimates. Finally, and not 

surprisingly, OLS is downward biased. However, the magnitude of the bias is invariant to the 

fraction of zero observations in the data. 

It is worth reiterating that, when the sample is restricted to doers, the reason for zeros in 

time-diary data is the mismatch between the length of the reference period (the diary day) and 

the period over which decisions are made. Thus, it stands to reason that lengthening the 

reference period should reduce the fraction of zero observations. A study by Foster and 

Kalenkoski (2008) examines how the diary window length affects OLS and Tobit estimates. 

Their findings are generally consistent with my results. They find that Tobit marginal effects are 

smaller than OLS estimates but that the difference is not that large. This is consistent with my 

finding that Tobit marginal effects are downward biased while OLS estimates are unbiased, and 

that the bias associated with Tobit marginal effects is small as long as the fraction of zero 

observations is not too large. My calculations based on their Table 2 indicate that the fraction of 

zeros is between 0.35 and 0.39 for the 48-hour window and between 0.38 and 0.43 for the 24-

 

11 In the first set the constant was set to 1 (vs. 10), which resulted in 22 percent of the sample being non-doers. The 

corresponding numbers in the second set were −2 and 42 percent. 
- 14 -

hour window. In this range, I find that Tobit marginal effects are fairly close to OLS estimates. 

They also argue that Tobit marginal effects are more sensitive to window length than OLS 

estimates, but the differences between the two sets of coefficients do not appear to be statistically 

significant. This is not too surprising given that the difference in the fraction of zeros is not that 

different for the two window lengths. 

Given the robustness of OLS to alternative assumptions about the data-generating process 

and the ease of estimating OLS, it is hard to recommend either Tobit or the two-part model. If 

the researcher is interested in the likelihood of engaging in the activity on a given day the twopart model, though unpredictable, outperforms Tobit. But for most policy-related questions, it is 

only necessary to know how certain covariates affect the average amount of time spent in an 

activity and the added information about the probability of engaging in the activity on a given 

day adds little. 
- 15 -

Appendix: Algorithms for generating zero-value observations

 For each respondent, observations are sorted by ehd and assigned a rank, Rh(ehd). The 

lowest value of ehd is ranked 1, the second lowest is ranked 2, etc. Values of ehd are set to zero if 

Rh(ehd) ≤ Th, where Th is determined as follows: 

Figure 1: The number of zero days is unrelated to the value of h c or any of the xi. 

Th = round(U(0,1) ×  ) 

Figure 2: The number of zero days is negatively related to the value of h c . 

Th = round(U(0,1) ×  ) − round(U(0,1) × h c ) 

Th = round(2 × U(0,1) ×  ) − round(3 × U(0,1) × h c )

Th = round(3 × U(0,1) ×  ) − round(3 × U(0,1) × h c ) 

Figure 3: The number of zero days is negatively related to the value of x1. 

Th = round((4 – xh1) × U(0,1) ×  ) 

Figure 4: The number of zero days is positively related to the value of x2. 

Th = round(0.5 × (3 + xh2) × U(0,1) ×  ) 

Figure 5: The number of zero days is negatively related to the value of x3. 

Th = round((2 − xh3) × U(0,1) ×  ) 

In each set of simulations, the parameter  was initially set to 0 set to 0 and incremented by 1 in 

each subsequent simulation until the percent of zero-value observations in the entire sample 

reached 90 percent (so that ݍത ranged from 0 to 0.9). Note that values of Th < 0 are treated the 

same as values of 0. 
- 16 -

References

Amemiya, Takeshi (1973) “Regression Analysis when the Dependent Variable is Truncated 

Normal.” Econometrica 41(6), November 1973), pp. 997-1016. 

Blundell, Richard and Costas Meghir (1987) “Bivariate Alternatives to the Tobit Model.” 

Journal of Econometrics 34, pp. 179-200. 

Cragg, John G. (1971) “Some Statistical Models for Limited Dependent Variables with 

Application to the Demand for Durable Goods.” Econometrica 39(5), Sept. 1971, pp. 

829-844. 

Daunfeldt, Sven-Olov and Jörgen Hellström (2007) “Intra-household Allocation of Time to 

Household Production Activities: Evidence from Swedish Household Data.” Labour

21(2), pp. 189-207. 

Flood, Lennart and Urban Grasjo (2001) “A Monte Carlo Simulation Study of Tobit Models.” 

Applied Economics Letters 8, pp. 581-584. 

Foster, Gigi and Charlene M. Kalenkoski (2008) “Tobit or OLS? An Empirical Evaluation Under 

Different Diary Window Lengths.” Unpublished manuscript, Ohio University. 

Frazis, Harley and Jay Stewart (2009) “How Does Household Production Affect Measured 

Income Inequality?” forthcoming in Journal of Population Economics. 

Hamermesh, Daniel S. (2009) “Grazing and Making Fat: Determinants and Effects.” NBER 

Working Paper No. 15277, August 2009. 

Keen, Michael (1986) “Zero Expenditures and the Estimation of Engel Curves.” Journal of 

Applied Econometrics 1(3), July 1986, pp. 277-286. 

Kimmel, Jean and Rachel Connelly (2007) “Mothers’ Time Choices: Caregovomg. Leisure, 

Home Production, and Paid Work.” Journal of Human Resources 42(3), Summer 2007, 

pp. 643-681. 

Kalenkoski, Charlene, David Ribar, and Leslie Stratton (2005) “Parental Childcare in SingleParent, Cohabiting, and Married-Couple Families: Time-Diary Evidence from the United 

Kingdom.” American Economic Review Papers and Proceedings 95(2), May 2005, pp. 

194-198. 

McDonald, John and Robert Moffitt (1980) “The Uses of Tobit Analysis.” The Review of 

Economics and Statistics 62(2), May 1980, pp. 318-321. 

Price, Joseph (2008) “Parent-Child Quality Time: Does Birth Order Matter?” Journal of Human 

Resources 43(1), pp. 240-265. 

Souza-Poza, Alfonso, Hans Schmid, and Rolf Widmer (2001) “The Allocation and Value of 

Time Assigned to Housework and Child-care: An Analysis for Switzerland.” Journal of 

Population Economics 14, pp. 599-618. 

Stewart, Jay (2009) “The Timing of Maternal Work and Time with Children.” Unpublished 

manuscript, Buearu of Labor Statistics. 

Tobin, James (1958) “Estimation of Relationships for Limited Dependent Variables.” 

Econometrica 26(1), Jan. 1958, pp. 24-36. 
Table 1: Mean Squared Error for Alternative Procedures by Fraction of Zero‐Value Observations

x1 x2 x3

OLS Tobit

2‐Part

Model OLS Tobit

2‐Part

Model OLS Tobit

2‐Part

Model

1 q ≤ 0.2 0.00 0.04 0.02 0.00 0.13 0.07 0.02 0.11 0.08

0.2 < q ≤ 0.4 0.00 0.11 0.00 0.00 0.45 0.00 0.01 0.25 0.01

0.4 < q ≤ 0.6 0.01 0.39 0.01 0.00 1.20 0.00 0.03 0.65 0.02

0.6 < q ≤ 0.8 0.02 0.69 0.02 0.01 2.74 0.01 0.07 1.18 0.06

q > 0.8 0.04 1.10 0.03 0.02 4.26 0.02 0.11 2.01 0.10

2a q ≤ 0.2 0.00 0.01 0.00 0.00 0.05 0.00 0.01 0.03 0.01

0.2 < q ≤ 0.4 0.00 0.03 0.00 0.00 0.14 0.00 0.01 0.07 0.02

0.4 < q ≤ 0.6 0.01 0.11 0.01 0.00 0.36 0.01 0.03 0.19 0.03

0.6 < q ≤ 0.8 0.03 0.27 0.02 0.01 0.98 0.01 0.08 0.47 0.07

q > 0.8 0.07 0.46 0.07 0.02 1.71 0.01 0.15 0.82 0.15

2b q ≤ 0.2 0.00 0.01 0.01 0.00 0.05 0.01 0.01 0.03 0.01

0.2 < q ≤ 0.4 0.00 0.00 0.01 0.00 0.02 0.01 0.01 0.01 0.03

0.4 < q ≤ 0.6 0.01 0.02 0.01 0.00 0.03 0.01 0.04 0.07 0.03

0.6 < q ≤ 0.8 0.03 0.08 0.03 0.01 0.18 0.02 0.13 0.16 0.13

q > 0.8 0.05 0.17 0.05 0.01 0.50 0.02 0.10 0.24 0.09

2c q ≤ 0.2 0.00 0.01 0.01 0.00 0.05 0.00 0.01 0.02 0.01

0.2 < q ≤ 0.4 0.00 0.01 0.01 0.00 0.01 0.02 0.02 0.02 0.03

0.4 < q ≤ 0.6 0.01 0.02 0.02 0.01 0.04 0.02 0.04 0.06 0.03

and Ranges for Fraction

of Zero Observations

Corresponding Figure

0.6 < q ≤ 0.8 0.02 0.07 0.02 0.01 0.16 0.01 0.05 0.13 0.06

q > 0.8 0.05 0.16 0.05 0.02 0.47 0.02 0.18 0.34 0.17

3 q ≤ 0.2 0.00 0.08 0.00 0.00 0.13 0.00 0.02 0.12 0.02

0.2 < q ≤ 0.4 0.01 2.63 0.01 0.00 0.56 0.01 0.01 0.29 0.02

0.4 < q ≤ 0.6 0.02 17.59 0.01 0.00 1.50 0.01 0.03 0.70 0.03

0.6 < q ≤ 0.8 0.04 64.10 0.11 0.02 3.30 0.26 0.11 1.34 0.13

q > 0.8 0.06 85.71 0.65 0.03 4.83 0.66 0.26 1.96 0.31

4 q ≤ 0.2 0.00 0.04 0.00 0.00 0.04 0.00 0.03 0.12 0.03

0.2 < q ≤ 0.4 0.01 0.12 0.01 0.00 0.02 0.01 0.01 0.28 0.01

0.4 < q ≤ 0.6 0.01 0.33 0.01 0.00 0.45 0.04 0.02 0.61 0.03

0.6 < q ≤ 0.8 0.03 0.78 0.04 0.00 1.44 0.01 0.07 1.42 0.09

q > 0.8 0.04 1.06 0.04 0.02 1.35 0.01 0.12 1.92 0.10

5 q ≤ 0.2 0.00 0.04 0.00 0.00 0.13 0.00 0.02 0.07 0.03

0.2 < q ≤ 0.4 0.01 0.13 0.01 0.00 0.48 0.00 0.02 1.76 0.03

0.4 < q ≤ 0.6 0.01 0.35 0.01 0.00 1.27 0.00 0.06 15.16 0.43

0.6 < q ≤ 0.8 0.03 0.79 0.03 0.01 2.83 0.01 0.06 29.07 0.53

q > 0.8 0.04 1.11 0.04 0.02 4.38 0.03 0.14 33.51 0.24
Figure 1: The Fraction of Zero Observations is Independent of Variables in the 

Model

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 2a: The Fraction of Zero Observations is Negatively Related to the 

Amount of Time Spent in the Activity

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 2b: The Fraction of Zero Observations is Negatively Related to the 

Amount of Time Spent in the Activity

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 2c: The Fraction of Zero Observations is Negatively Related to the 

Amount of Time Spent in the Activity

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 3: The Fraction of Zero Observations is Negatively Related to x1

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 4: The Fraction of Zero Observations is Positively Related to x2

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3
Figure 5: The Fraction of Zero Observations is Negatively Related to x3

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

OLS 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Tobit Marginal Effects 

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

50%

0 0.2 0.4 0.6 0.8

Fraction Zero Observations

Two-Part Model Marginal Effects 

x1 x2 x3