Monthly Labor Review • May 2009 57

Workplace Injuries and Illnesses

Comparing Workers’ Compensation claims 

with establishments’ responses to the SOII

Nicole Nestoriak 

and

Brooks Pierce The Bureau of Labor Statistics’ Survey of Occupational Injuries and 

Illnesses (SOII) collects and tabulates employer reports on work-related injuries and illnesses. SOII estimates are the 

primary source of information on nonfatal 

work-related injuries and illnesses in the 

United States.

Recent work comparing SOII microdata with other administrative sources of 

work-related injury and illness data, in 

particular Workers’ Compensation (WC) 

claims databases, concludes that the SOII

substantially undercounts cases. This article focuses on the paper “Capture-Recapture Estimates of Nonfatal Workplace 

Injuries and Illnesses” by Leslie I. Boden 

and Al Ozonoff, which compares SOII 

case records with WC microdata for several States. Their findings indicate that the 

SOII detects between 50 percent and 75 

percent of cases in the States studied.1

The present article describes the 

Boden-Ozonoff study and reports some 

additional findings that were obtained 

by analyzing a subset of the data that the 

Boden-Ozonoff paper used. This new 

research extends the aggregate results reported by Boden and Ozonoff in order to 

determine which types of cases the SOII

is most likely to undercount. In particular, 

the present article focuses on differences in 

the SOII capture rate by establishment type, by 

time of case filing, and by type of injury.

Methods

The basic method underlying the BodenOzonoff study involves comparing the SOII list 

of injury and illness cases with an analogous 

list, covering the same workforce, from the 

Workers’ Compensation administrative system 

to determine to what extent the lists overlap. 

Cases in the WC claims microdata that are 

not found in the SOII sample are considered 

missed by the SOII and form the basis of the 

estimated SOII undercount.

Although this method is logically straightforward, it is difficult to carry out. Because 

any given injury is processed independently 

and represented differently in the two systems, it is not always possible to definitively 

link the case’s representation in the SOII with 

its representation in the WC data. Further, it 

is often difficult to determine whether or not 

a reported WC injury or illness case occurred 

in an establishment that was within the SOII 

sample. Finally, in comparing the SOII and WC

data it is critical to exclude cases outside the 

scope of one or the other of the data sources; 

otherwise a simple difference in scope will be 

misinterpreted as underreporting.2 It is someNicole Nestoriak and Brooks 

Pierce are economists in the 

Office of Compensation and 

Working Conditions, Bureau of 

Labor Statistics. E-mail: nestoriak.nicole@bls.gov, pierce.

brooks@bls.gov.

Comparing elements of the Workers’ Compensation database 

with data from the Survey of Occupational Injuries and Illnesses

 is a useful way to determine which types of injuries and illnesses 

the SOII is most likely to undercount 
Workplace Injuries and Illnesses

58 Monthly Labor Review • May 2009

times challenging, however, to determine whether or not 

a given case is in scope for WC or the SOII. 

Data sources

This section describes the data sources for the article and 

describes in particular the aspects of the data relevant to 

the matching exercise.

The SOII is an annual establishment survey that most 

recently sampled approximately 176,000 establishments 

in private industry. Because the SOII is a survey, it does not 

give a complete listing of the experiences of every privatesector establishment. Rather, sampled establishments in 

effect represent the greater universe of establishments. 

Sampling is a valid approach for producing estimates, but 

the fact that the SOII is based on a sample rather than 

a census does make the matching exercise in Boden and 

Ozonoff ’s paper more challenging.

SOII respondents are directed to report from on-site 

injury logs maintained as part of the Occupational Safety 

and Health Administration’s record-keeping requirements. The record-keeping rules dictate that records be 

maintained at an establishment’s physical location; accordingly, BLS samples data at the establishment level 

rather than at the firm level.3 Firms with multiple sites 

or establishments may have some, none, or all of their establishments sampled in any given year. Data for a given 

survey year are reported to BLS in the first half of the year 

following the survey year.

For more serious injury or illness cases—those involving at least 1 day away from work beyond the date of 

injury or onset of illness—the SOII collects detailed information describing the incident and the affected employee. 

The SOII program refers to these cases as “days away from 

work” cases. The information that is collected includes the 

nature and source of the injury or illness, the part of the 

body affected, and the date of the onset of the injury or illness, as well as the employee’s name, date of birth, sex, and 

race. These data, as well as information on the employer, 

are used to help identify cases for the purposes of matching SOII records with WC administrative records.

The Boden-Ozonoff group obtained permission from 

several States to match WC claims microdata with SOII 

microdata. Because the SOII data are confidential, all data 

analysis was carried out at BLS. And because WC data include confidential information, there were some data to 

which BLS did not have access. However, BLS did obtain 

permission from one State, Wisconsin, to further analyze 

its 1998–2001 WC data. Boden and Ozonoff also made 

their intermediate data sets available to BLS, which made 

this article’s detailed analysis possible.

Workers’ Compensation systems differ from State to 

State, but on the whole they have similar features. Most 

States mandate coverage of nearly all private-sector workers. WC typically covers almost all medical expenses arising from a work-related injury or illness, it recompenses 

portions of lost earnings due to temporary injuries or illnesses if the duration of the injury or illness exceeds a 

minimum waiting period, and it provides partial or total 

disability payments in the event of permanent injury or 

illness. Temporary injury and illness cases in Wisconsin 

from 1998 to 2001 were compensable under WC if they 

satisfied a 3-day waiting period. An employee generally 

has 2 years to report a workplace injury to his or her employer, although most injuries are reported much earlier. 

Some traumatic injuries (vision loss, total loss of a hand or 

arm, permanent brain injury, etc.) and some occupational 

diseases (carpal tunnel syndrome, hearing loss, etc.) have 

no time limit for filing a claim.

Under the WC system, cases may be claimed by workers 

but disputed by the employer.4 An employer may believe 

a given injury is not work related, or the employer may 

dispute the degree of disability. In such cases the employee may request that the State office of WC resolve the 

dispute via a hearing before an administrative law judge. 

Negotiated settlements are possible. The WC data that this 

article uses include some contested cases and negotiated 

settlements, but they are not identified separately from 

the other cases. The Wisconsin WC system reported on 

average about 50,000 lost-time claims per year over the 

2000–06 period. Of these, about 18 percent (an annual 

average of about 9,200 claims) were marked as denials, 

as injuries or illnesses that that did not require days away 

from work, or as noncompensable cases. About 13.6 percent (6,800) of claims were litigated annually.5

The Boden-Ozonoff study imposes scope restrictions 

on each data source; the intent is for every data source to 

refer to the same sets of at-risk private-sector employees. 

As an example, mining and railroad sector data are excluded because the SOII program does not collect those 

data through its normal survey instrument. (Rather, it 

relies on administrative files from the Mine Safety and 

Health Administration and the Federal Railroad Administration.) As another example, injury and illness cases 

in the SOII involving fewer than 3 days away from work 

are excluded, as such cases do not meet the Wisconsin 

WC system waiting period requirements.6 Perhaps the 

most important scope restriction calls for the discarding 

of WC cases that arose in establishments which are not 

in the SOII sample. To do so accurately requires that one 
Monthly Labor Review • May 2009 59

Table 1. Capture propensities by status of establishments, 1998–2001

 

Total number of cases............................................... 121,567 77,967 17,798

Percent of cases captured by the SOII.................. 77.5 62.2 52.8

Percent of cases captured by Workers’ ............... 79.7 83.3 84.0

 Compensation

Single-establishment firms

Table 1.

NOTE: The “percent captured” rows show the percentage of observed cases captured by the Survey of Occupational Injuries and Illnesses and by Workers’ Compensation. Data are calculated using Workers’ Compensation cases from single-establishment firms in Wisconsin.

Establishments within

multiestablishment firms

Establishments within

firms of unknown status

identify the establishments from the SOII sample in the 

WC data, which may be difficult—especially in the case of 

multiestablishment firms as described earlier. In general 

one expects such scope restrictions to cause some degree 

of error beyond the margin of error that would normally 

be expected. Some of the numerical results in this article 

are consequently subject to some additional error because 

of issues of scope caused by data limitations in the BodenOzonoff study.

In the end, there are 4 years of SOII and WC injury and 

illness case data available. These data comprise approximately 217,000 distinct cases.7

 The SOII and WC case lists 

overlap substantially, but not completely: the SOII list covers about 70 percent of all observed SOII and WC cases, 

and the WC list covers about 81 percent. In other words, 

the Boden-Ozonoff study suggests that the SOII estimates 

undercount observed cases by about 30 percent.8

Single-establishment and multiestablishment

firms

Whereas the SOII data come from establishments chosen 

for the sample, the WC data tend to reflect reporting by 

firms. Consequently, the WC data are not detailed enough 

to allow one to consistently determine where within firms 

injuries and illnesses have occurred. The issue is a problem 

when a firm has multiple establishments of which only 

some are sampled by the SOII. Is an injury case apparently 

missed by the SOII truly a missed case, or rather is it an injury that occurred at an establishment not in the sample? In 

this circumstance there is some ambiguity about whether 

to treat the case as one that was misreported to the SOII. 

The Boden-Ozonoff study recognizes this issue and 

makes a statistical adjustment in the instances in which it 

arises. Nevertheless, because the issue is an important one, 

it makes sense to show separate results for single-establishment and multiestablishment firms. The data, when 

organized in this way, show that the SOII appears to miss 

more cases in multiestablishment firms. This may be due 

to an intrinsic difference between single-establishment 

and multiestablishment firms, or it may result from the 

method used for matching.

Table 1 presents statistics by establishment status. Of 

the cases in either the Wisconsin WC data or the SOII

data, roughly 56 percent are in single-establishment firms 

and 36 percent are in multiestablishment firms. The remaining 8 percent are of unknown status because there is 

not enough information available to label them as either 

single-establishment or multiestablishment firms.9

Table 1 shows that the SOII capture rates are higher 

when only single-establishment firms are considered: 

according to the calculations, the SOII captures 77.5 

percent of the estimated cases in this subset of the data. 

The SOII’s rate of capture of injuries and illnesses in multiunit establishments is 62.2 percent. In establishments 

of unknown status, the capture rate is 52.8 percent. The 

data for establishments of unknown status appear to behave—both here and in other tabulations—more like the 

multiestablishment than the single-establishment data. 

One possible explanation for the differences in capture 

rates across establishment types is that the single-establishment firms actually do not report their behavior in 

the same way that establishments in multiestablishment 

firms do. Note, however, that the WC capture rate is similar across the establishment types. Thus, there appears to 

be some particular reporting or measurement effect that 

differs by establishment status within the SOII but not 

within the WC administrative system. 

Another possibility is that the single-establishment 

firm subset of the data yields more accurate estimates because the method used to adjust the multiestablishment 

results introduces error. For the single-establishment firm 
Workplace Injuries and Illnesses

60 Monthly Labor Review • May 2009

Table 2. SOII capture propensity by year of WC filing, 1998–2001

 

Same year as survey year........................................... 83,256 86.0 76.1

1 year after close of survey year.............................. 12,406 12.8 48.0

2 years after close of survey year............................ 917 .9 19.2

3 years after close of survey year............................ 203 .2 4.9

Table 2.

NOTE: Data are calculated using Workers’ Compensation cases from single-establishment firms in Wisconsin.

Year of WC case filing Number of cases Percent of WC cases SOII capture propensity

subset of the data, it is rarer to encounter ambiguity concerning whether or not a given WC claim case occurred 

in an establishment sampled by the SOII. Distinguishing 

between these two possibilities is an important topic for 

further study.

The remainder of this article focuses on cases involving 

single-establishment firms. Although these cases do not 

represent the full spectrum of cases, using only data from 

single-establishment firms allows one to avoid situations 

in which one does not know whether an observed WC case 

is within the SOII sample or not. Restricting the sample in 

this manner is akin to restricting the scope of the two data 

sources in the hope that each data source refers to the 

same set of workers and injury and illness cases.

SOII capture propensity by time of WC filing

The timing of the collection of injury and illness data is 

another characteristic that differs between WC and the 

SOII, and it may explain part of the undercount. The SOII

collects data in the first 6 months of the year following 

the year of incidence and only contains cases that are recognized as valid, work-related cases of injuries or illnesses 

that occurred during or just after the survey year. Cases 

that are not recognized prior to data collection obviously 

are not included in the SOII counts. The WC administrative data, however, cover cases that were recorded up to 2 

years following the date of incidence.

The extract of the Wisconsin WC data used in this article does not include a list of cases’ filing dates. However, 

the WC system assigns case identifiers sequentially, and 

the case identifier embeds the year of the filing. From the 

case identifier one can therefore generate a year and an 

imputed month of filing for cases in the WC system.10 Out 

of the 121,567 cases in single-establishment firms that the 

SOII captured, 96,884 cases are also in the WC system and 

are used in this analysis. The remaining 24,683 are in the 

SOII records but not the WC records, and they therefore 

will be dropped from the remainder of this analysis as, 

by definition, there is no time-of-WC-filing information 

available for these cases. 

Table 2 shows case counts and the “SOII capture propensity” as functions of the year of the WC filing. SOII’s 

“capture propensity” is defined here as the percent of WC

cases that appear in the SOII. A case with a date of injury 

in 1998 and a WC system identifier indicating a filing in 

2000 would be included in the row “2 years after close of 

survey year.” Note that about 12.8 percent of cases are filed 

in the year following the survey year. We refer to these as 

“1-year-after” data for simplicity. A little over 1 percent of 

cases are filed with a greater lag. The final column shows 

the SOII capture propensity. 

Two broad facts are clear in these data. First, there are 

a substantial number of cases filed under the WC program 

after the close of the SOII survey year. Second, the SOII 

capture propensity is much lower for these particular cases. Together these facts suggest that the WC data include 

many cases that are not known to SOII respondents, or 

have not been deemed work related, at the time of the 

survey response.

Aside from the year of filing, another known fact is 

the order in which cases are entered into the WC system. 

Cases in the 1-year-after data occur disproportionately 

early in the filing sequence. About half of these cases appear to have been filed early in the calendar year following 

the SOII survey year. For that half, the SOII capture rate is 

fairly high, approximately 60–65 percent. For the other 

half of the 1-year-after data, the SOII capture rate is approximately one-third. Thus, the 1-year-after capture rate 

of 48.0 in table 2 masks variation within the year.

One reasonable conclusion to make is that about half 

of the 1-year-after filings are either: 1. delayed WC filings 

from workers in establishments that replied to the SOII

with accurate responses, 2. injury and illness cases that 
Monthly Labor Review • May 2009 61

occurred late in the year and were known to SOII respondents at the time they responded, or 3. a combination of 1 

and 2. The remaining half of the 1-year-after filings may 

reflect continuing or late-developing lost-workday cases 

attributed to past injuries. There also exist other possibilities, such as reconciled disputes that enter the books late. 

While the SOII program would obviously like to collect 

information on all workplace injuries and illnesses occurring in the survey year, the completeness of the data 

needs to be weighed against the timeliness in generating 

statistics.

Other indicators of low SOII capture propensity 

The results in the previous section indicate that some WC

injury and illness cases are reported well after the close of 

the survey year, and this raises the question of whether or 

not these cases are identifiably different in the WC system. 

In other words, are they recognized by the WC system as 

distinct from the cases reported within the survey year? 

The WC system maintains a variety of fields used to aid in 

administration. Some of these fields have data that correlate with the data that are reported late, and this correlation may help in understanding some of the difficulties in 

matching administrative data from the WC system with 

survey data from the SOII.

To understand WC system data, it helps to understand 

WC filing requirements. If an injury or illness results in 

days away from work beyond Wisconsin’s 3-day waiting 

period, the employer or its insurer must file a first report of 

injury within 7 days of onset. The first report contains basic information on the employee and the injury or illness. 

The employer or its insurer must also file a supplementary 

report within 30 days of onset. This supplementary report 

either indicates the amount and type of WC payments to 

the employee—including whether the payments are for 

temporary total or temporary partial disability—or otherwise must indicate a claim denial or investigation. Additional supplementary reports must be filed as payments 

are changed—for example, because of a change in status 

from temporary to permanent disability—or stopped, 

usually by the employee’s return to work. The WC data 

system generates a status flag on the basis of the initial 

supplementary report, which typically captures payment 

information soon after the onset of the injury or illness. 

As shown in table 3, there are clear differences in the SOII

capture propensity across status flag values. 

The WC data system maintains information on days 

of Total Temporary Disability (TTD), information that is 

based on the cumulative supplementary report filings for a 

given claim. A day of temporary total disability is roughly 

analogous to a lost workday in the SOII. Although the 

data are restricted to lost-workday cases in this analysis, 

many of the claims have a TTD-day value of zero in the 

WC system.11

In an analysis conducted for this article, it was found 

that cases reported late tend to have a disproportionately 

high number of atypical status flag values and a disproportionately high number of cases with zero TTD days 

recorded. It was also found that SOII capture propensity 

tends to vary by WC status flag and by the incidence of 

zero TTD days recorded, even among WC cases reported 

prior to the close of the survey year.

Table 3 shows some of the relevant statistics. The table 

displays SOII capture rates, the prevalence of zero-TTDday cases, median case durations, and WC filing lags, all by 

WC status flag. The average WC filing lag is based on the 

imputed month of filing, as discussed previously. “Case 

Table 2. SOII capture propensity and other case characteristics, by Workers’ Compensation status flag, 1998–2001

 

 Total........................................................ 71.8 11.8 10 2.1 96,884

Award........................................................ 20.2 89.8 0 7.8 1,787

Electronic................................................. 67.5 9.5 10 1.7 15,986

Final ........................................................... 74.2 10.1 10 1.9 78,145

Under Investigation............................. 0 0 142 3.0 7

Not final.................................................... 37.4 38.1 4 11.6 12

No lost time............................................. 44.2 32.1 3 6.3 833

Not required ........................................... 13.0 100.0 0 19.8 97

Table 3.

SOII capture 

propensity

Percent with zero 

TTD days

Average filing lag

 (in months) WC status flag Number of cases

Median case

 duration 

 (in days)

NOTE: Data are calculated using Workers’ Compensation cases from single-establishment firms in Wisconsin.
Workplace Injuries and Illnesses

62 Monthly Labor Review • May 2009

duration” refers to the number of days away from work 

due to the injury or illness in question.12 

About 97 percent of WC cases have a status flag of “electronic” or “final.” Cases marked as “final” have WC payment 

information included in the initial supplementary filing. 

A case marked as final is likely to be a rather typical case 

that has been provisionally recognized by the employer. 

Cases marked as “electronic” are those filed electronically; 

unfortunately, there is little else that this status flag reveals 

about cases. Cases marked as “final” or “electronic” are not 

expected to be especially unusual as a group. These cases 

are on average reported relatively promptly to the office 

that handles WC claims, and they have typical durations. 

Of the remaining 3 percent of WC cases, the majority have the “award” status. Cases marked as “award” are 

those for which a formal order has been written providing 

compensation for the claim. Cases with award status are 

typically disputed cases adjudicated in the claimant’s favor 

or settled by the claimant and the employer’s insurer. The 

SOII only captures 20 percent of the cases with an “award” 

status code. When a case is disputed, the final determination of whether the injury or illness is work related can occur long after the year of injury and can result in a lumpsum payment without distinguishing the number of TTD

days involved. This reasoning is consistent with the fact 

that about 90 percent of award-status cases have zero TTD

days recorded. The cases with zero TTD days were likely 

not perceived as recordable cases by the SOII respondents 

at the time of the survey. The status code “no lost time” 

indicates the case was initially coded as having no lost 

workdays. Consequently, a case coded as “no lost time” can 

be one that did not involve days away from work prior 

to the initial supplementary report but did involve lost 

workdays afterward. The category of no lost time is small, 

and cases in the category tend to have low SOII capture 

rates, shorter durations than average, and some lag in reporting. 

One of the main points of table 3 is that in the WC system, both the type of injury or illness case and the length 

of time between the onset of the injury or illness and the 

filing date of the case are related to the likelihood of the 

case being reported to the SOII. Certain cases or case types 

are less likely to be captured by the SOII. The SOII probably misses some cases that it should have captured, but 

because of difficulty in determining which cases are in and 

out of scope, some of the cases that the SOII is found to 

“miss” actually could be cases that are outside its scope. 

In order to provide more clarity, the next section of the 

article documents the types of injury and illness cases that 

are more likely to be reported to the SOII.

Better detection of some injuries and illnesses 

than others 

Both the SOII and WC databases contain information on 

the broad type of injury or illness relevant to each case. 

This information is referred to as the “nature” of the case, 

and it identifies the principal physical characteristics of 

the injury or illness. It is easy to imagine that some case 

types are easier to identify in general, or are easier to identify specifically as work related, or are more likely to be 

perceived as severe and therefore presumably more likely 

to be reported in the SOII or in WC claims.

Table 4 shows the most common nature-of-injury-orillness codes in the WC administrative data, ranked in descending order by the SOII capture propensity.13 Like table 

3, table 4 also reports the percent of cases with zero TTD 

days reported, median case durations, and the average WC

filing lag.

Categories within the nature-of-injury-or-illness column that cover problems one could reasonably view as 

severe, easily identifiable, or having a sudden onset tend 

to be better captured by the SOII. For example, the capture 

propensities for amputation cases and severance cases are 

both about 90 percent. At least according to these data, 

the vast majority of amputations are reported in the SOII. 

Cases involving concussions, fractures, punctures and the 

like also tend to have relatively high SOII capture rates. 

Case types such as lacerations, contusions, and strains, 

in which one might expect somewhat greater heterogeneity of severity or ease of identification, tend to show average SOII capture rates. Given that these kinds of injuries 

are quite common, documenting sources of heterogeneity 

within this subset of cases is expected to be an important 

element of future research.

Injuries that become apparent or worsen over time such 

as inflammation or carpal tunnel are reported in the SOII

much less frequently than the average injury or illness. 

These case types also tend to show longer-than-average 

lags between the onset of the injury or illness and the WC

filing. Presumably, some of these cases develop too late for 

inclusion in the SOII’s collection of data; alternatively, the 

cases may be reported less often to the SOII because of 

greater difficulty in determining whether or not they are 

work related. 

Note that the SOII appears to capture virtually zero of 

the hearing loss cases. These cases tend to have long reporting lags and are overwhelmingly reported as having 

zero TTD days. SOII respondents may not believe these 

injuries and illnesses to be recordable by the Occupational 

Safety and Health Administration, or they may simply 
Monthly Labor Review • May 2009 63

Table 1. SOII capture propensity and other case characteristics, by nature of injury or illness, 1998–2001

 

 Total.............................................................. 71.8 11.8 10 2.1 96,884

Amputation................................................... 90.6 13.7 11 1.6 858

Severance ...................................................... 90.0 6.6 13 1.1 122

Dislocation .................................................... 88.4 5.6 12 1.4 414

Foreign body ................................................ 87.5 7.4 5 1.4 410

Multiple physical injuries ........................ 84.4 10.1 14 1.7 2,080

Fracture .......................................................... 82.8 8.9 18 1.2 6,846

Burn ................................................................. 82.5 5.6 6 .9 1,322

Infection......................................................... 82.3 21.7 7 1.7 143

Puncture......................................................... 82.0 6.9 6 .9 676

Concussion.................................................... 81.9 2.7 7 .9 149

Hernia.............................................................. 79.5 3.3 16 2.7 2,481

Crushing......................................................... 79.0 12.2 12 1.2 1,243

Dermatitis...................................................... 76.4 40.5 10 3.3 304

Sprain.............................................................. 75.2 8.0 8 1.3 4,937

Laceration...................................................... 75.2 10.8 9 1.6 5,285

Contusion ...................................................... 73.3 8.2 8 1.5 5,773

Strain ............................................................... 70.9 11.9 9 1.9 45,296

Other specific injuries............................... 69.1 10.6 10 2.5 10,941

Respiratory disorders................................ 60.3 19.6 6 3.0 147

Rupture........................................................... 58.5 12.7 25 6.4 461

Carpal tunnel syndrome........................... 58.4 9.3 24 4.6 2,649

Inflammation................................................ 57.3 13.2 12 2.5 1,266

Other cumulative injuries........................ 51.1 24.4 9 4.1 1,620

Loss of hearing............................................. 7.4 94.1 0 10.4 714

Hearing loss (traumatic)........................... 0 100.0 0 11.4 167

Table 4.

SOII capture

 propensity Nature of injury or illness

Percent of cases 

with zero 

TTD days

Median case

duration

(days away

from work)

Average filing lag

(in months) Number of cases

NOTE: Data are calculated using Workers’ Compensation cases from single-establishment firms in Wisconsin.

not know they exist at the time of report. Additionally, 

these may be cases for which employees have stronger incentives to file a WC claim, as hearing aids are not covered 

by most health insurance plans.

The general patterns in table 4 suggest that the SOII does 

a very good job of capturing certain classes of cases, but 

they also suggest that the SOII fails to capture a noticeable 

fraction of cases—a quarter or more—within certain frequently occurring case types such as strains and sprains. It 

is possible that differences in circumstances among similar injuries and illnesses within these categories influence 

measurability. If such an underlying heterogeneity exists, 

identifying it would be a useful step toward understanding the root causes of the estimated SOII undercount.

THE PURPOSE OF THIS ARTICLE IS TO SHOW some of 

the dimensions of the estimated SOII undercount. The 

patterns of variation of the SOII capture rate shown in 

tables 1–4 suggest various possible explanations for the 

undercount. It may be that certain types of cases are inherently difficult to identify as work related, especially in a 

timely manner. Further, there may be some yet-unknown 

differences in scope between WC and the SOII. As an example, some of the WC cases with zero reported TTD days 

may be cases with no lost worktime, cases which by design 

should not appear in the SOII as days-away-from-work 

cases. Finally, a precise matching of cases from these two 

different databases may require data that are better suited 

for matching than those currently available. That is, some 

of the estimated undercount may be due to outstanding 

methodological issues that are difficult to resolve absent 

finer data. Clearly, there are various hypotheses that have 

been proposed with the aim of explaining the discrepancies between the WC and SOII databases. These hypotheses will need to be scrutinized and tested further in order 

to achieve a full understanding of the differences between 

the ways in which the WC and SOII systems measure 

workplace injuries and illnesses.
Workplace Injuries and Illnesses

64 Monthly Labor Review • May 2009

NOTES

1

 Leslie I. Boden and Al Ozonoff, “Capture-Recapture Estimates of Nonfatal Workplace Injuries and Illnesses,” Annals of Epidemiology, June 2008, pp. 

500–06. See also John Ruser, “Examining evidence on whether BLS undercounts 

workplace injuries and illnesses,” Monthly Labor Review, August 2008, pp. 

20–32.

2

 Boden and Ozonoff are aware of such difficulties in comparing the SOII

and WC data and take great effort to account for them in their calculations. 

When making the less straightforward calculations in their study, they often 

purposefully err on the side of producing a smaller estimate of the SOII undercount. Of course, the SOII and WC data were not designed in anticipation of 

comparing them, and one should therefore expect some data-related problems 

to remain.

3 To construct the SOII sampling frame, BLS takes all units within scope for 

the SOII from a universe of establishments that report on unemployment insurance. BLS then makes some improvements to this sampling frame on the basis 

of historical collection experience. The intent is to construct a frame of physical 

establishment locations; however, in some cases firms are Statewide reporters, 

in which case they file only one report in each State in which they operate, and 

the report covers all their establishments in the State. Firms sometimes also 

have other ways of filing one report that covers multiple establishments (and 

therefore multiple physical locations).

4

 For evidence on incentives to report injuries to the WC program, see Jeff 

Biddle and Karen Roberts, “Claiming Behavior in Workers’ Compensation,” 

The Journal of Risk and Insurance, December 2003, pp. 759–80.

5

 See www.dwd.state.wi.us/wc/WC_Basic_Facts.htm#WC_Claim_and_

Indemnity_Information (visited May 1, 2009).

6

 There are also situations in which the SOII, to ease respondent burden, 

collects data for only a subset of the cases occurring in a reporting establishment. Other times—though rarely—not all establishments of a given firm are 

actually able to provide data on all their cases of injuries and illnesses; this often occurs because the boundaries of establishments can be unclear. In such a 

situation, some sampled units are permitted to provide data that cover more or 

fewer employees than are officially in the establishment. When, for any of the 

aforementioned reasons, the SOII does not have data on all the cases in a given 

establishment, weighting adjustments enable the SOII to statistically account for 

all cases. However, these situations require further scope adjustments for the 

purposes of matching SOII data to WC data. The Boden-Ozonoff study makes 

scope adjustments for establishments from which the SOII, to ease respondent 

burden, collects data for only a subset of injury and illness cases. It is not able to 

make scope adjustments when sampled units are permitted to provide data that 

cover more or fewer employees than are officially in the establishment.

7

 All case totals in this article are weighted totals that are calculated using 

SOII sampling weights.

8

 The Boden-Ozonoff study imputes that approximately 24,000 cases are 

missed by both the WC and SOII systems, and the authors report that the SOII

undercount is larger than that of the WC program. The statistics presented here 

do not include imputations for cases missed by both surveillance systems.

9

 An establishment is identified as part of a multiestablishment firm if there 

are multiple establishments within the same unemployment insurance reporting number during the survey year. This method of identification oversimplifies 

because firms can encompass business lines across more than one unemployment insurance reporting number. The establishments of unknown status have 

unemployment insurance reporting numbers that exist in the sampling frame 

at the time the sample is drawn, but not during the survey year. These establishments either merged with other establishments, went out of business, or were 

otherwise redefined in the sampling frame at some point between the date of 

the drawing of the sample and the survey year.

10 This is done by assuming a constant rate of filing over the course of the 

year. That is, a claim with an assigned claim number in the bottom fourth of the 

distribution of numbers is imputed to have been filed in the first quarter. The 

imputed-month-of-filing data are not error free, but they do correlate well with 

the date-of-injury-or-illness data recorded in the BLS system for matched cases. 

The imputation is therefore believed to be useful.

11 There are a number of scenarios that can lead to a claim being marked 

as having zero TTD days in the WC administrative data. As one example, the 

employer can continue regular salary payments to an employee whose injury 

or illness has caused days away from work, such that no compensation for lost 

earnings is due to the employee. As another example, the insurer can erroneously make WC payments (which would initiate a claim in the system) though 

the waiting period has not been satisfied. Another possibility is for compromise 

settlements to be recorded as having no compensable TTD days due. One cannot 

determine the reason that a given case has been designated as a zero-TTD case, 

but the scenarios noted here suggest that these cases are probably more difficult 

to capture in the SOII. Cases that truly involve no lost workdays, such as cases of 

an immediate permanent disability upon injury, are presumably excluded from 

these data.

12 The number of days away from work is reported consistently to the SOII. 

For cases that are in the WC database but not in the SOII database, the number 

of days away from work is imputed using TTD days. 

13 The nature-of-injury-or-illness codes used in the Wisconsin WC system 

differ from the codes used by BLS in its publications. Therefore, cases identified as, for example, punctures in table 4 would not necessarily be identified as 

punctures under the BLS categorization.