Nutrition Assistance Program Report Series 

The Office of Analysis, Nutrition and Evaluation 

Special Nutrition Programs Report No. WIC-03-WICVM 

 A Comparison of WIC Vendor 

Management Practices in 

1991 and 1998 

United States Food and 

Department of Nutrition 

Agriculture Service 

May 2003
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A Comparison of WIC Vendor 

Management Practices in 

1991 and 1998 

Authors:

Anita Singh 

Patricia McKinney 

John Endahl 

Dawn Aldridge 

Jay Hirschman 

Submitted by: 

Office of Analysis, Nutrition and Evaluation 

USDA, Food and Nutrition Service 

3101 Park Center Drive, Room 1014 

Alexandria, VA 22302-1500

This study is an in-house analysis by Food and Nutrition Service Staff. 

This report is available on the Food and Nutrition Service website: http://www.fns.usda.gov/oane.

Suggested Citation:

Anita Singh, Patricia McKinney, John Endahl, Dawn Aldridge and Jay Hirschman. “A Comparison of WIC 

Vendor Management Practices in 1991 and 1998,” Nutrition Assistance Program Report Series, No. WIC02-WICVM. U.S. Department of Agriculture, Food and Nutrition Service, Office of Analysis, Nutrition and 

Evaluation, Alexandria, VA: 2003. 

United States Food and 

Department of Nutrition 

Agriculture Service 

May 2003

Special Nutrition Programs

Report No. WIC-03-WICVM
Table of Contents 

 Page 

I. Introduction and Overview 1

II. Data Analyses 4 

III. Findings 4 

IV. Summary of Findings 9 

V. References 11 

VI. List of Figures 12 
1

I. Introduction and Overview: 

The Food and Nutrition Service (FNS) published “The WIC Vendor Management Study, 1998” 

in July 2001 which examined, in part, the extent to which retail grocers, defined as “vendors” 

in the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC), were 

violating program rules and regulations. The 1998 study is a follow-up to the “WIC Vendor 

Issues Study, 1991” published by FNS in May 1993. From an operational and management 

perspective, it is important for FNS to know if there have been any changes in vendor 

management practices from 1991 to 1998. However, as there were differences in the way the 

data were collected, analyzed, and reported, the findings presented in the two published reports 

cannot be directly compared. This report presents a re-analysis of the data from the 1991 and 

1998 studies, which allows comparisons of the findings. 

 

 Similarities as well as differences between the two studies are presented in Table 1. Major 

differences between the two studies include the manner in which compliance buys were 

conducted and the data analyzed. A compliance buy is an investigative technique wherein 

covert buyers, equipped with WIC food instruments, attempt to make WIC purchases at stores 

selected for the study. Four types of compliance buys were made: 1) safe buy in which the 

buyer attempted to purchase all items listed on the WIC food instrument in the exact quantity 

specified; 2) partial buy in which the buyer attempted to purchase some but not all items listed 

on the WIC food instrument; 3) minor substitution buy in which the buyer attempted to 

substitute an unauthorized food item within an approved food category; and 4) major 

substitution buy in which the buyer attempted to purchase an unauthorized item clearly outside 

an approved food category (see Table 1). In 1991, three “safe” buys were conducted for all 

vendors, but in 1998, only one “safe” buy was conducted (see Table 1). Furthermore, the 

percentage of vendors committing procedural and administrative errors and undercharging or 

overcharging the Program in the 1991 study is based on noting an occurrence during any one 

of the three safe buys. This type of data analysis resulted in higher percentages for the errors 

and program violations studied than those obtained by averaging across the three safe buys or 

by examining one safe buy. For example, “WIC Vendor Issues Study, 1991” reported that 

22.2% of all vendors overcharged the Program at least once during the three safe buys (see 

Appendix A), whereas in this re-analysis, using only the first safe buy, 9.9% of vendors 
2

overcharged the Program. The 1991 report did not present averages across the three safe buys. 

Thus, to compare the rates of vendor administrative errors and violations in 1991 and 1998, it 

was necessary to re-analyze the two data sets. 

Table 1. Similarities and Differences in Characteristics of the 1991 and 1998 Samples 

Characteristics 1991 1998 

Study Population Includes vendors from 46 of the 48 

contiguous States, plus 

Washington, DC, all of which have 

retail food delivery systems. Also, 

includes pharmacies and drug stores 

(2.2% of the sample)1

. 

Excludes Mississippi and Vermont. 

Also, excludes Indian Tribal 

Organizations and military 

commissaries.

Includes vendors operating in 

States with retail food delivery 

systems plus Washington DC. 

Excludes Mississippi, Vermont, 

North Dakota, and parts of Ohio 

and Illinois, military commissaries 

and pharmacies. Also, excludes 

Alaska, Hawaii, Puerto Rico, U.S. 

Territories and Indian Tribal 

Organizations. 

Sampling frame Vendors were selected using a twostage probability selection: Primary 

sampling units (PSUs) then vendors 

within PSUs. PSU was defined as 

WIC vendors contained in a group 

of one or more contiguous counties 

in the same state. Each PSU had at 

least 80 vendors. 

The U.S. was divided into 60 strata. 

One PSU per strata was selected. 

About 20 vendors were selected 

from each PSU. 

Over-sampled other than large 

chain stores relative to large chain 

stores in order to test the hypothesis 

that small to medium-sized 

“independent” vendors abuse the 

Program at higher incidence than 

major chain stores. The overall 

sampling rates for these groups 

were set so they did not vary by 

more than 3:1. 

Vendors had participated in the 

Program for at least 12 months. 

Used Geographic Information 

System (GIS) computer program to 

form 366 PSUs in contiguous 

geographic areas. Most PSUs had at 

least 70 vendors. Selected 100 

PSUs using probability nonreplacement sampling with 

probabilities proportional to the 

size of the PSU. Selected about 18 

vendors each from the 100 PSUs. 

 

Over -sampled PSUs at the rate of 

2:1 from Vendor-Specific Food 

Instrument (FI) States versus States 

that operate an Open FI system. 

Length of Program participation 

varied and could not be ascertained 

from the data files that were 

submitted to FNS. 

 

1 Note: Pharmacies and drugstores could not be excluded from the sample for the re-analysis because these vendors were not identified 

separately when the data files were created and submitted to FNS.
3

Sample Size Nationally representative sample. 

Total 1,205 (unweighted) vendors 

weighted up to 34,033 vendors. 

58% were from a metropolitan area. 

Nationally representative sample. 

Total 1,600 (unweighted) vendors 

weighted up to 36,754 vendors. 

72% were from a metropolitan area. 

Definition of Vendor Size Small = 1 to 5 cash registers 

(accounted for 75% of the sample). 

Large = 6 or more cash registers 

(accounted for 25 % of the sample.) 

Small = 2 or fewer cash registers; 

Medium = 3 to 7 cash registers; and 

Large >8 or more cash registers. 

Redefined for this report as Small 

= 1 to 5 cash registers (51% of the 

sample) and large = 6 or more cash 

registers (49% of the sample.) 

Compliance Buy Methodology Three safe buys (Buy 1, 2 and 3) 

were conducted at all vendors. The 

sample was then divided into 3 

groups of 400 vendors each. The 

1st group received two additional 

safe buys for a total of 5 safe buys. 

The 2nd group did not receive any 

more compliance buys. The 3rd

group received a major substitution 

buy wherein the buyer attempted to 

purchase a non-WIC food item. 

During all safe buys, the buyer 

attempted to purchase all items 

listed on the WIC food instrument 

in the exact quantity specified. 

Three buys (Buy # 1, Buy # 2 and 

either Buy # 3A or 3B) were 

conducted at each vendor: 

Buy # 1 = safe buy (buyer 

attempted to purchase all food 

listed on the food instrument in the 

quantities and types specified.) 

Buy # 2 = partial buy (buyer 

attempted to purchase some, but not 

all of the items listed on the food 

instrument.) 

Buy # 3A = minor substitution 

(buyer attempted to substitute an 

unauthorized food item within an 

approved food category.) 

Buy # 3B = major substitution 

(buyer attempted to substitute an 

unauthorized item clearly outside 

an approved food category) 

Presentation of findings in the 

two published reports. 

Percentages are based on 

occurrence on at least one of three 

safe buys. Not an average of the 

three safe buys. Also presented 

data on major substitution buy. 

Re-analysed for this report and 

compared first safe buy in 1991 to 

first buy in 1998 (safe buy).

Only one safe buy – the first buy. 

Data for partial buy, minor and 

major substitution buys were also 

presented. 

In this re-analysis, the first buy in 1998, which was the only safe buy, was compared to the 

first safe buy in 1991. Only one safe buy was conducted in 1998 because the 1991 study did 

 
4

not find a familiarization effect across the 3 safe buys. Specifically the following comparisons 

were made: 

1. The extent to which vendors commit procedural and administrative errors when 

conducting a WIC transaction at the point of sale. 

2. The extent to which vendors overcharge and undercharge the WIC Program. 

3. The extent to which vendors allowed participants to substitute unauthorized items for 

authorized items on the food instrument. 

II. Data Analyses: 

Raw data sets provided by the contractors, who had conducted the 1991 (Aspen Systems 

Corporation) and the 1998 (Health Systems Research, Inc.) studies for FNS, were re-analyzed 

using the Statistical Analysis System (SAS version 8). The first safe buy in 1991 was 

compared to first and only safe buy in 1998. Frequencies were computed on data that had been 

weighted. Chi-squares were computed on weighted data to test for statistical significance. The 

level of significance was set at p<0.05. 

III. Findings: 

In the following sections, a comparison of administrative errors, rates of overcharging and 

undercharging, as well the percentage of vendors accepting major substitutions, are presented. 

A. Comparison of Administrative Error Rates 

In both studies, similar information was collected on two types of administrative errors, 

insufficient stock and improper recording of the purchase price at the time of the transaction. 

For each of these two variables, information was analyzed by vendor size (small and large) and 

by type of food package (woman, infant, and child). As previously stated, the data were reanalyzed and comparisons were made between the first safe buy in 1991 and the only safe buy 

in 1998. 
5

1. Insufficient Stock 

Most States require vendors to maintain sufficient stocks of WIC-authorized foods to ensure 

that participants can purchase their prescribed foods in one visit. Insufficient stock violations 

include not having sufficient stock or correct brand or size of a particular WIC-authorized food 

item to fill a participant’s food instrument. As shown in Figure 1, there was a significant 

decrease in the overall percentage of vendors with insufficient stocks of WIC-authorized foods 

from 8.8% in 1991 to 4.6% in 1998. 

 

Findings related to insufficient stock by food package type are shown in Figure 2. There was a 

statistically significant (p<0.001) decrease in the percentage of vendors who were not meeting 

stocking requirements for the woman and child food packages. The percentages decreased 

from 10% in 1991 to 2.6% in 1998 for the woman package and from 9% in 1991 to 1.7% 1998 

for the child package. In contrast, significantly more vendors in 1998 (9.3%) were out-ofstock on items in the infant food package as compared to 1991 (7.9%). 

Figure 3 displays the percentage of vendors who did not meet stocking requirements by vendor 

size. In 1991 and 1998, more small vendors than large vendors did not have sufficient stock. 

Overall the percentages of small and large vendors that were not meeting stocking 

requirements were significantly (p<0.001) lower in 1998 as compared to 1991. 

2. Purchase Price Procedures 

In both studies, violating the countersignature regulations was a predictor of vendor abuse. 

The 1991 study measured whether the purchase price was entered on the Food Instrument (FI) 

at the time of the transaction. The 1998 study measured whether the vendor deviated from 

countersignature regulations by either requiring the buyer to sign the FI prior to the cashier 

entering the purchase price or if the buyer was not asked sign the FI. In the 1998 study, there 

was a response category “don’t know.” Vendors’ responses coded under “don’t know” 

accounted for 3% of all the responses for the nationally representative 1998 sample and were 

excluded from the analysis. 
6

Figure 4 displays the percentage of WIC vendors who did not follow proper procedures for 

entering the price at the time of sale by type of food package. As stated above vendor 

responses that were coded under “don’t know” in the 1998 sample were excluded from the 

analysis (less than 3 percent of the sample was excluded). As shown in Figure 4, the 

percentage of vendors who did not record the price on the FI for the woman food package 

significantly (p<0.001) decreased from 12% in 1991 to 9.5% in 1998. The percentage of 

vendors who did not follow the correct procedures for recording the purchase price for the 

infant and child packages in 1991 and 1998 were similar. 

Both studies examined the occurrences of vendors not recording the purchase price by vendor 

size. As previously stated vendor responses that were coded under “don’t know” in the 1998 

sample were excluded from the analysis. Thus, 3.6% of small vendors and 1.5% of large 

vendors were excluded from the analyses. As shown in Figure 5, the percentage of small and 

large vendors not following purchase price recording procedures was similar in 1991 and 1998. 

In 1991 and 1998 large vendors demonstrated lower occurrences of violations compared to 

small vendors. 

B. Vendor Overcharge and Undercharge 

An overcharge occurred if the vendor redeemed more than the current, verified shelf price for 

WIC-authorized foods that were purchased with food instruments. Whether a vendor’s 

overcharges were deliberate actions or error was not determined. An undercharge was noted if 

the vendor redeemed a food instrument at less than the shelf price for the WIC-authorized 

foods provided. An overall distribution of vendors who overcharged, undercharged, and 

correctly charged the Program during the first safe buy in 1991 and the only safe buy in 1998 

is presented in Figure 6. The percentage of vendors who did not overcharge or undercharge 

WIC increased from 81.8% in 1991 to 86% in 1998. Overcharging and undercharging are 

discussed in the following sections. 

1. Overcharging by Vendors 

The percentage of vendors who overcharged the Program decreased significantly (p<0.001) 

from 9.9% in 1991 to 7% in 1998. The overcharge rate was also analyzed by vendor size (see 

Figure 7). In both 1991 and 1998, a greater percentage of small vendors overcharged as 
7

compared to large vendors. However, the percentage of small and large vendors who 

overcharged the Program decreased significantly (p<0.001) from 1991 to 1998. 

As shown in Figure 8, the rate of overcharging was also analyzed by location to determine if 

there were differences in overcharging between vendors in metropolitan areas versus those in 

non-metropolitan areas. The percentage of metropolitan and non-metropolitan vendors who 

overcharged decreased significantly (p<0.001) from 1991 to 1998. 

In Figure 9, the comparison of overcharging in 1991 and 1998 by type of food package is 

presented. In this analysis the woman and child packages were combined. The percentage of 

vendors overcharging the Program decreased significantly (p<0.001) from 1991 to 1998 for the 

woman and child as well as for the infant package. 

2. Undercharging by Vendors 

When comparing undercharging on the first safe buy in 1991 to the only safe buy conducted in 

1998, the percentage of vendors undercharging the Program decreased significantly (p<0.001) 

from 1991 to 1998 (8.3 % vs. 7.0 %) (see Figure 6). As shown in Figure 10, the data also were 

analyzed to determine if vendor location, metropolitan versus non-metropolitan, affected the 

rate of undercharging. Significantly (p<0.001) fewer metropolitan vendors in 1998 (6.1%) 

undercharged the Program as compared to 1991 (8.1%). In contrast, the rate for undercharging 

by non-metropolitan vendors increased, though not significantly, from 8.5% in 1991 to 9.3% in 

1998. 

3. Estimating Net Cost to WIC of Overcharging and Undercharging by Vendors

In 1991, the total federal dollar loss from overcharging was the total estimated dollar amount 

lost during one year among all authorized retail vendors. Estimates for overcharging and 

undercharging were calculated for each vendor in the WIC vendor universe. The data were 

weighted to generate national estimates for overcharging and undercharging. Unlike the 1991 

study, the 1998 study was not weighted to generate a dollar amount to use to calculate a true 

national estimate for the net cost to the Program. Given the differences in the methodology for 

generating the overcharging and undercharging estimates, it is not possible to directly compare 

the dollar estimates of the 1991 study to the 1998 study. The estimated amount and rate of 
8

overcharging and undercharging during first safe buy in 1991 and the only safe buy in1998 are 

presented below. However, as stated above, the differences in the methodology for generating 

these estimates preclude any comparisons of the 1991 and 1998 findings for overcharging and 

undercharging. 

Based on the first safe buy in 1991, the estimated cost of overcharging was $ 30.73 million 

with 1.47 percent of WIC redemptions, nationally, attributed to overcharging. The 

corresponding estimated cost for undercharging was $ 9.98 million with a rate of 0.5 percent 

nationally. Thus, the estimated net loss to WIC from overcharging and undercharging errors 

was $20.75 million. 

The 1998 study reported that the estimated rate for overcharging was 0.9 percent and the rate 

of undercharging was 0.4 percent during the only safe buy that was conducted. Recently, 

when responding to requests to estimate the costs to the Program in 2001, the Food and 

Nutrition Service (FNS) applied rates from the 1998 study for estimating the net loss to WIC 

from overcharging and undercharging errors. Using the rate of overcharging based on all three 

buys (1 safe, 1 partial, and 1 substitution buy), it was estimated that 1.6 percent of 1998 WIC 

redemptions nationally were attributed to overcharging. Using only the safe buy to calculate 

the estimate, the percentage drops to 0.9 percent. Using the rate of undercharging based on all 

three buys, it was estimated that 0.6 percent of 1998 WIC redemptions nationally were 

attributed to undercharging; whereas, using only the safe buy to calculate the estimate, the 

percentage of undercharge drops to 0.4 percent. Therefore, based on fiscal year 2001 annual 

retail redemptions of $4.48 billion and applying the rate of 0.9 percent for the safe buy and 1.6 

percent for all three buys, FNS estimated the cost of overcharging to the Program to range 

from about $40 million to $72 million. After accounting for undercharging at the rate of 0.4 

percent for the safe buy to 0.6 percent for all three buys the estimated range for undercharging 

is about $18 million to $27 million. Based on these estimates, FNS estimated that the net loss 

to WIC from overcharging and undercharging errors ranges between $22 million and $45 

million. 

C. Vendors Allowing Major Substitutions

In this section, a comparison is made between the percentage of vendors who allowed 
9

compliance buyers to substitute an unauthorized item for a WIC-authorized one. As indicated 

in Table 1, there were differences in the method in which major substitution buys were 

conducted in 1991 and 1998. A larger sample of vendors received a major substitution 

compliance buy in 1998 than in 1991. In 1991, only 396 vendors or about one third of the 

sample were visited, but in 1998, 747 vendors were studied. In 1991, all the major substitution 

compliance buyers attempted to purchase one six-pack of soda to replace one item on the food 

instrument. In 1998, compliance buyers were provided with a list of unauthorized foods, such 

as soda, pretzels, cheese doodles, and canned pasta, and were instructed on which item to 

substitute. Overall findings, as well as percentages by vendor size and location, are presented 

and discussed below. 

Unlike 1998, in 1991 a separate vendor weight was not developed for the major substitution 

buy. Therefore, the comparisons shown in Figures 11 and 12 are made on unweighted data. 

Although the overall percentage of vendors who allowed major substitutions to be made is 

small (1.5% in 1991 and 4.0% in 1998), the percentage was significantly (p<0.001) higher in 

1998 compared to 1991 (see Figure 11). The percentage of small and large vendors who 

allowed compliance buyers to substitute WIC-authorized foods also was significantly 

(p<0.001) higher in 1998 than in 1991 (see Figure 11). The highest percentage (5.5%) was 

noted for small vendors in 1998. 

In Figure 12, data are shown by vendor location, metropolitan versus non-metropolitan. A 

significantly (p<0.001) higher percentage of metropolitan vendors in 1998 (4.3%) allowed a 

major substitution than in 1991 (0.9%). The percentage of non-metropolitan vendors who 

committed this violation in 1998 (3.1%) was also higher than in 1991 (2.3%), but the 

difference was not significant. 

IV. Summary of Findings: 

Overall, the percentage of vendors committing administrative and procedural errors and 

mischarging the Program declined from 1991 to 1998. 

• More vendors maintained sufficient stocks of WIC-authorized foods for women and 
10

children in 1998 than in 1991. The reverse was true for the infant package wherein the 

percentage of vendors with insufficient stock was significantly (p<0.001) higher in 1998 

than 1991 (9.3% versus 7.9%). 

• In 1991 and 1998, small vendors were more likely than large vendors to have insufficient 

stocks of WIC-authorized foods. Nonetheless, the percentage of small and large vendors 

with insufficient stocks declined significantly (p<0.001) from 1991 to 1998. 

• Some improvements were noted in the percentage of vendors who did not record the 

purchase price at the time of the transaction. The percentage of vendors committing this 

error on the woman package receipts decreased from 1991 to 1998. However, over 13% of 

small vendors and over 5% of large vendors in 1991 and 1998 failed to record the purchase 

price at the time of the transaction. 

• Accuracy in charging WIC improved from 1991 to 1998. The percentage of vendors 

overcharging or undercharging the Program decreased significantly (p<0.001). 

• In 1991 and 1998, small vendors were more likely to overcharge WIC than large vendors. 

Overcharging was also more prevalent in metropolitan areas than non-metropolitan areas. 

• Although the percentage of vendors allowing the compliance buyer to substitute an 

unauthorized item for a WIC-authorized food item was low, this practice increased 

significantly (p<0.001) from 1991 to 1998. A higher percentage of small and metropolitan 

vendors allowed a major substitution than large or non-metropolitan vendors. 

Finally, the findings of this comparison study indicate that, overall, vendor management 

practices improved between 1991 and 1998. Notable improvements were noted in the 

maintenance of sufficient stocks of WIC-authorized foods by vendors and in vendor accuracy 

of charging the Program. An area that did not show improvement in 1998 was the practice of 

accepting buyer-initiated substitutions for WIC-authorized foods. Although the percentage of 

vendors allowing substitution buys is small, it is an area that needs improvement. 
11

V. References:

1. U.S. Department of Agriculture, Food and Nutrition Service, Office of Analysis and 

Evaluation. WIC Vendor Issues Study, 1991: Final Report, by Lilly B. Gardner, et. al. Project 

Officers, Jay Hirschman, Ken Offerman, Steven Gale. Alexandria, VA: 1993. 

2. U.S. Department of Agriculture, Food and Nutrition Service, Office of Analysis, 

Nutrition, and Evaluation. WIC Vendor Management Study, 1998: Final Report, by Loren 

Bell, et.al. Project Officers, Patricia McKinney, Boyd Kowal. Alexandria, VA: 2001. This 

report can be downloaded from 

http://www.fns.usda.gov/oane/MENU/Published/WIC/WIC.HTM
12

VI. List of Figures: 

Figure 1: Comparison of the National Estimate of the Percentage of WIC Vendors with 

Insufficient Stock 

Figure 2: Comparison of the National Estimate of the Percentage of Vendors with Insufficient 

Stock by Type of Food Package 

Figure 3: Comparison of the National Estimate of the Percentage of Vendors with Insufficient 

Stock by Size of Vendor 

Figure 4: Comparison of the National Estimate of the Percentage of Vendors Who Did Not 

Record the Purchase Price at the Time of Transaction by Type of Food Package 

Figure 5: Comparison of the National Estimate of the Percentage of Vendors Who Did Not 

Record the Purchase Price at the Time of Sale by Size of Vendor 

Figure 6: Comparison of the National Estimate of the Percentage of WIC Vendors who 

Overcharged, Undercharged and Correctly Charged the WIC Program 

Figure 7: Comparison of the National Estimate of the Percentage of Vendors Who 

Overcharged the WIC Program by Size of Vendor 

Figure 8: Comparison of the National Estimate of the Percentage of Vendors Who 

Overcharged the WIC Program by Location 

Figure 9: Comparison of the National Estimate of the Percentage of Vendors Who 

Overcharged the WIC Program by Type of Food Package 

Figure 10: Comparison of the National Estimate of the Percentage of Vendors Who 

Undercharged the WIC Program by Location 

Figure 11: Vendors Who Accepted A Buyer-Initiated Major Substitution Buy: Comparison of 

the Overall Percentage of Vendors and by Vendor Size 

Figure 12: Comparison of the Percentage of Vendors Who Accepted A Buyer-Initiated Major 

Substitution by Location 
Figure 1

Comparison of the National Estimate of the Percentage of W

Insufficient Stock

1991 1998

8.8% 4.6%

*p<0.001 1991 versus 1998

Sufficient Stock Insufficient Stock

1991 
Figure 2

Comparison of the National Estimate of the Percentage of Vendors 

by Type of Food Package

0123456789

10

Woman Infant Child

Type of Food Package

10.0

2.6*

9.3*

1

7.9

9.0

*p<0.001 1991 versus 1998

Percentage of Vendors
Figure 3

Comparison of the National Estimate of the Percentage of Vendors 

by Size of Vendor

02468

101214

Small Large

Vendor Size

12.1

6.9*

2.3*

*p<0.001 1991 versus 1998

Percentage of Vendors 3.6
Figure 4

Comparison of the National Estimate of the Percentage of Vendor

the Purchase Price at the Time of Transaction by Type of 

02468

1012

Overall Woman Infant Ch

Type of Food Package

12.0

9.5*

8.3

7.9

11.7

*p<0.001 1991 versus 1998

Percentage of Vendors

10.5

9.6*
Figure 5

Comparison of the National Estimate of the Percentage of Vendors 

the Purchase Price at the Time of Sale by Size of V

02468

101214

Small Large

Vendor Size

13.6 14.0

5.2

Percentage of Vendors 5.6
Figure 6

Comparison of the National Estimate of the Percentage of W

Overcharged, Undercharged and Correctly Charged the W

1991 1998

8.3%*

7.0%

Charged Correctly Undercharged Overcharged

9.9%*

81.8%

7.0%

*p<0.001 1991 versus 1998

1991 
Figure 7

Comparison of the National Estimate of the Percentage of Vendors 

WIC Program by Size of Vendor

02468

101214

Small Large

Vendor Size

13.3

10.9*

3.0*

Percentage of Vendors 4.6

*p<0.001 1991 versus 1998
Figure 8

Comparison of the National Estimate of the Percentage of Vendors 

WIC Program by Location

02468

1012

Metropolitan Non-Metropolitan

Location of Vendor

10.2

7.7*

5.3*

Percentage of Vendors

9.5

*p<0.001 1991 versus 1998
Figure 9

Comparison of the National Estimate of the Percentage of Vendors 

WIC Program by Type of Food Package

02468

1012

Woman and Child Infant

Type of Food Package

10.9

7.5*

6.0*

Percentage of Vendors 8.0

*p<0.001 1991 versus 1998
Figure 10

Comparison of the National Estimate of the Percentage of Vendor

the WIC Program by Location

0123456789

10

Metropolitan Non-Metropolitan

Vendor Location

8.1

6.1*

9.3

Percentage of Vendors

8.5

*p<0.001 1991 versus 1998
Figure 11

Vendors Who Accepted A Buyer-Initiated Major Substitution Buy

Overall Percentage of Vendors and by Vendor S

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Overall Small Vendors Large Vend

1.7

5.5*

Percentage of Vendors (Unweighted) 1.0

*p<0.001 1991 versus 1998

4.0*

1.5
Figure 12

Comparison of the Percentage of Vendors Who Accepted A Bu

Substitution by Location

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Metropolitan Non-Metropolitan

Vendor Location

0.9

4.3*

3.1

Percentage of Vendors 2.3

*p<0.05 1991 versus 1998