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Validity of Children's Food Portion Estimates
A Comparison of 2 Measurement Aids
Donna M. Matheson, PhD;
Kara A. Hanson, RD;
Tiffany E. McDonald, MPH;
Thomas N. Robinson, MD, MPH
Arch Pediatr Adolesc Med. 2002;156:867-871.
ABSTRACT
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Background Policy and clinical decisions regarding children's nutrition are often
based on dietary intake estimates from self-reports. The accuracy of these
estimates depends on memory of both the type of food eaten and the amount
consumed. Although children's self-reports of food intake are widely used,
there is little research on their ability to estimate food portions.
Objective To assess the validity of children's estimates of the food portions
they consume by means of 2 types of measurement aids: standard 2-dimensional
food portion visuals and manipulative props.
Design Randomized controlled trial.
Participants Fifty-four African American girls aged 8 to 12 years.
Main Outcome Measures Girls were served a standard meal and actual intake was assessed by
weighing food portions before and after the meal. On completion of the meal,
dietitians collected food recalls and portion size estimates from the girls
by means of both manipulative props and 2-dimensional food portion visuals,
administered in a randomized order.
Results Absolute value percentage differences between actual and estimated grams
of food consumed averaged 58.0% (SD, 102.7%) for manipulative props and 32.8%
(SD, 72.8%) for 2-dimensional food portion visuals. Spearman correlations
between actual and estimated intakes with both portion size measurement aids
were high (range, r = 0.56 to 0.79; all P<.001), with the exception of bread intake (r = 0.16, P = .43). Correlations with actual
intakes did not differ significantly between the 2 methods.
Conclusions Children's self-reported portion size estimates are appropriate for
ranking children's relative intakes, but they result in sizable errors in
quantitative estimates of food and energy intakes. Caution should be used
in interpreting quantitative dietary intake estimates derived from children's
self-reports.
INTRODUCTION
MANY CLINICAL and policy decisions and recommendations regarding childhood
nutrition are based on dietary intake estimates derived from children's self-reports,
such as 24-hour recalls or food frequency questionnaires.1-10
Therefore, their usefulness depends, in part, on the validity of these estimates.
There are 2 primary sources of potential errors in self-reports of dietary
intake: remembering the types of foods eaten and the amounts of foods consumed.
Previous research has focused more on memory of the types of foods eaten.
This research has demonstrated that preadolescent children are able to remember
between 50% and 84% of the types of foods that they consumed during the previous
24 hours.3, 5, 11 There
has been less research on the accuracy of children's recalls of the amount
of food they consumed.5, 12
There is no research that validates children's use of common food portion
measurement aids.13 In adults, the validity
of a variety of food portion measurement aids, including 2- and 3-dimensional
models, has been tested, but this research has not produced conclusive results.13 There have been no consistent differences in the
errors associated with any of the food portion measurement aids tested. Average
errors in adults' portion size estimates have ranged from 23% to 63%. In general,
errors associated with overestimates of food portions are larger than errors
in underestimation.14 Similar data do not exist
for children. Therefore, to examine the validity of children's self-reports
of food portions, we compared children's measurements of actual food consumption
with estimates produced from self-reports by means of 2 types of food portion
measurement aids: manipulative props and 2-dimensional food models.
SUBJECTS AND METHODS
SUBJECTS
This study was conducted as part of the formative research for the Stanford
Girls Health Enrichment Multisite Studies project, an obesity prevention study
for African American girls. African American girls aged 8 to 12 years were
recruited from 7 community centers and church groups in the San Francisco
Bay area of California. Eight groups, of 4 to 11 girls each, participated
in focus groups during which they were served a standard dinner. All procedures
were explained to parents and their daughters before they participated in
the project. The girls were told that they would be served weighed portions
of food, they did not need to consume all the food that was served to them,
and they could ask for additional servings of any of the foods provided. They
were also told that their plates and cups would be reweighed after they were
finished eating and they would be asked to estimate how much food they had
consumed. Parents provided signed written consent for their child's participation
and the girls provided assent. All girls who attended the focus groups participated
and completed the portion size experiment. The study was approved by the Panel
on Human Subjects in Medical Research at Stanford University, Palo Alto, Calif.
PROCEDURES
An experiment was designed to compare children's actual food intake
with estimates derived from 2 portion-size measurement aids. The girls were
offered standard portion sizes of 4 foods that represented all physical states
of food: solid, liquid, and amorphous (Table 1). They were not served any foods that they did not want
to eat. Before consumption, gram weights of all foods were obtained by means
of a digital kitchen scale (test-retest reliability intraclass correlation,
0.99). Children were monitored as they ate to ensure they did not share food
and that they returned their dishes, with all uneaten food, to the food preparation
area.
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Table 1. Portion Sizes Served and Food Models Used to Estimate Each
Food
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Within 10 minutes of finishing their meals, the girls were interviewed
by registered dietitians, who collected food recalls and portion size estimates.
The registered dietitians were all very experienced at obtaining food recalls
from children and followed standard procedures. First, the girls were asked
to recall the foods they consumed, and they were prompted for specific foods
that they ate but did not volunteer. The dietitian recorded foods that were
recalled with and without prompting. Because the focus of this study was on
portion size estimation, the dietitians were aware of the foods that were
served, but they were blinded to the amount of food the girls actually consumed.
Next, the girls were asked to estimate the amount of foods they consumed by
means of 2 portion-size measurement aids, the commonly used 2-dimensional
food portion visual developed by Nutrition Consulting Enterprises, Framingham,
Mass,15 and manipulative props that were craft
materials including modeling clay, small plastic beads, paper strips, and
water that girls used to demonstrate the amounts they consumed. The girls
completed their estimates of the amount of all the foods they consumed by
means of one of the aids before repeating their estimates with the other aid.
The girls were randomly assigned to one of the dietitians, who collected portion
size estimates with both measurement aids. The order in which the portion
size methods were administered was also randomly assigned for each girl. Each
method required approximately 5 minutes.
PORTION-SIZE MEASUREMENT AIDS
The 2-dimensional food portion visual developed by Nutrition Consulting
Enterprises has been compared with similar 3-dimensional models in samples
of adults. Correlations between the estimates produced by the 2-dimensional
and 3-dimensional models were highly significant (r
= 0.89 and r = 1.00 for men and women, respectively).16 Food portion estimates produced by these models have
not been compared with actual intakes.
We developed the use of manipulative props as a food portion measurement
aid. Therefore, this method has not previously been tested. The girls used
craft materials as props to demonstrate to the dietitian how much food they
consumed, and then the dietitian estimated or measured this amount in units
that could be entered into the NDS-R (Nutrition Data System for Research),
Version 4.02, dietary analysis program.17 Specifically,
the girls placed paper strips onto plates or bowls to demonstrate the amount
of spaghetti or salad they ate and the dietitian estimated this amount in
cups. Modeling clay was molded into a replica of the bread sticks and the
dietitian measured the dimensions. Water was poured from a 4-c (1-L) liquid
measuring cup into 1 of 3 glasses to indicate the amount of beverage consumed.
In comparison, the 2-dimensional food portion visuals are drawings of drinking
glasses with lines corresponding to standard servings (eg,  c [125
mL]) or shapes (eg, mounds, rectangles, wedges) in graduated sizes that represent
standard servings (eg, c and 3 oz [84 g]). Respondents select the
drawing that they believe best represents the amount of food they consumed.
The food portion measurement aids used for each food, under both experimental
conditions, are included in Table 1.
This method of food portion estimation in children was expected to have
4 advantages over other food portion measurement aids. First, recent research
indicates that visualization is one of the primary strategies used to recall
food intake.2, 18 The manipulative
props allowed children to visualize and physically alter the props to depict
the amount of food that they consumed. For example, they were able to show
the portion of food that was served to them and remove the amount that they
ate. Second, with the manipulative props, children's food portion estimates
would not be biased by the amounts depicted in the models, or limited by the
shape of food models that were made available. Third, a registered dietitian
was responsible for choosing the unit of measurement for the food, so children
were not required to describe food portions in ounces or by physical dimensions
as is often the case in standard food recalls. Fourth, children enjoy working
with the manipulative props and, therefore, their interest could be maintained
throughout the recall.
STATISTICAL ANALYSES
Descriptive statistics of the girls' weighed intakes and their food
portion estimates with the manipulative props and the 2-dimensional visuals
were calculated by means of the NDS-R software. Wilcoxon sign rank tests were
used to test the statistical significance of differences between actual intakes
and the estimates produced by the manipulative props or 2-dimensional food
portion visuals. Percentage errors were calculated by means of the absolute
values of the differences between actual and estimated portion sizes. Spearman
correlation coefficients were used to assess the level of association between
actual and estimated intakes derived from each portion size measurement aid.
A t test of the difference between dependent correlation
coefficients was used to test whether the correlations between the actual
intakes and manipulative prop estimates and the correlations between actual
intakes and 2-dimensional food portion visual estimates differed significantly
from each other.19
RESULTS
A total of 54 girls completed the experiment. Their ages ranged from
8 to 12 years, with a mean ± SD of 9.8 ± 1.1 years. All of the
girls who consumed spaghetti and salad recalled these foods. However, 14 (27%)
of 51 girls needed prompting to recall drinking a beverage and 6 (15%) of
39 girls required prompting to recall eating bread. Three girls each reported
eating one food (spaghetti, salad, or beverage) that they actually did not
consume. These phantom estimates were excluded from the analyses since percentage
error could not be calculated for actual intakes of zero. Actual food intake
and portion size estimates produced by the manipulative props and 2-dimensional
visuals and the differences between actual intake and the estimates are reported
in Table 2. Not all girls consumed
every food, and therefore the sample sizes differed slightly for each food.
Standard deviations and ranges in the portion-size estimation errors for most
foods were large, indicating that the girls' abilities to accurately estimate
food portions varied considerably. There were no statistically significant
differences due to the order in which the girls completed the portion size
estimates (P = .75) or due to the dietitian who conducted
the recalls (P = .22). The overestimates or underestimates
and percentage errors, based on the absolute value of the differences between
the estimated and actual intakes, are reported in Table 3. Most children overestimated their spaghetti intake and
underestimated their beverage intake. For other foods, there were no consistent
patterns across both methods in the number of girls who overestimated or underestimated
their intakes.
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Table 2. Actual Intakes and Estimated Food Portions
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Table 3. Average Percentage Overestimated or Underestimated by Each
Method and Average Percentage Error Based on the Absolute Value Difference
Between Actual and Estimated Food Intakes
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Spearman correlation coefficients between actual intakes and estimated
intakes are presented in Table 4.
With the exception of the correlation between actual intake of bread and the
estimate produced with manipulative props, all correlations were large20 and statistically significant (P<.001). The 2 methods did not produce statistically significantly
different correlations with either gram weights of actual intakes (P<.21) or the energy content of actual intakes (P<.21).
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Table 4. Spearman Correlations Between Actual Weight of Food Consumed
or Total Energy Intakes and Estimates Produced by Manipulative Props or 2-Dimensional
Food Portion Visuals
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COMMENT
In this study, we tested the validity of children's reports of the amount
of food they had just consumed. We also compared a novel portion-size measurement
aid for children with standard 2-dimensional food portion visuals. Overall,
the 2 estimates were of similar accuracy when compared with actual intakes.
Correlations between actual and estimated intakes derived from both measurement
aids were high,20 except for the bread estimates.
This result indicates that both methods were generally effective at ranking
girls' relative intakes. Girls who consumed larger portions of foods also
estimated larger portions of those foods. However, the correlations between
actual bread intake and the estimated intake were lower, particularly for
the manipulative props estimate. Modeling clay was the manipulative prop used
to estimate bread intake. It is possible that some girls may have enjoyed
playing with the modeling clay so much that they became less focused on the
purpose of the activity (ie, estimating their actual consumption) while using
this prop. Manipulative props are engaging for children and may be highly
effective for children who are unmotivated to complete dietary recalls; however,
manipulative props cannot be used over the telephone. Therefore, the data
collection protocols must be considered in deciding whether to use manipulative
props or 2-dimensional food portion visuals to collect portion size data from
children.
Because the estimates produced with both portion-size measurement aids
proved to be reasonably valid in ranking children's relative food intakes,
either of these methods may be used in large-scale correlational studies.
However, the magnitudes of the errors in children's quantitative estimates
of food portions were large. These results bring into question the validity
of the food and energy intakes derived from children's self-reports of dietary
intake. This is of concern especially since our study tested children's food
recalls and portion size estimates under optimal conditions; girls were informed
before eating that they would be asked to estimate their food intakes, and
the data were collected within 10 minutes of consuming their meal. Estimates
derived from a recall the next day, as standard 24-hour diet recalls are usually
performed, would be expected to have even greater errors.11
Basiotis and associates21 defined a precise
estimate of intake as being within 10% of the true intake for a sample of
people 95% of the time. In our research, absolute value errors in overall
energy intake averaged 54% for 2-dimensional visuals and 68% for the manipulative
props, well beyond this range. These large errors do not appear to be unique
to our sample. Other researchers also have reported errors well in excess
of 10%. In a recent experimental study in which third- and fourth-grade children
were asked to estimate known quantities of foods, percentage errors in absolute
values ranged from 25% for a prepackaged beverage to 539% for applesauce.12 Likewise, Lytle and associates5
reported that with the aid of 3-dimensional food models and tableware, children
were able to recall only 35% of foods within 10% of the observed portion sizes.
In that study, the portion sizes of approximately 42% of the foods were overestimated
and 23% were underestimated, and for many foods errors were in excess of 100%.
As with other studies, we found that errors due to overestimating food intakes
were larger than underestimates.
Previous validation research on children's dietary recalls has focused
on children's ability to remember both which foods and the amounts of foods
they consumed. In our research, we used an experimental design to isolate
the errors in children's ability to estimate the amount
of food they ate from the errors associated with remembering which foods they ate. Although more than one quarter of
the girls needed prompting to recall consuming beverages and/or bread, portion
size estimates were collected from all foods actually consumed by the children.
Therefore, the errors that we observed can be attributed to the girls' inability
to express accurate portion sizes, and not to their inability to remember
consuming a food. Additional errors in remembering which foods were eaten
and extending the time between consumption and the recall would be expected
to further increase the errors observed.22
Our research used an experimental design. Unlike many experiments in
which external validity is sacrificed, an attempt to maintain external validity
was made in this research. The girls ate in a group, similar to a school lunch
setting, and were able to refuse foods or ask for additional servings. The
foods served to the girls included solid, liquid, and amorphous foods. However,
with the exception of their beverage (milk, juice, or water), the girls did
not have a choice in the foods or the amounts that were served to them. Furthermore,
the amounts of spaghetti, salad, and beverage served to the girls were standard
serving sizes that directly corresponded to the size of mounds or drinking
glasses in the 2-dimensional food portion visuals. This would only be expected
to improve the accuracy of the girls' estimates with the 2-dimensional food
portion visuals, especially if the girls consumed all of the food served to
them. However, these estimates still averaged errors of greater than 100%.
The accuracy of the girls' reports of their food portions was highly
variable. Some girls were able to precisely estimate their food intake and
reported errors of less than 1% in the gram amount of a single food, while
others had errors in excess of 3000%. The variability observed in children's
ability to accurately estimate portion sizes may be related to their level
of cognitive development. Piaget suggested that 7- to 11-year-old children
are beginning to acquire concrete operational thought processes.23
These thought processes—conservation, for example—enable children
to quantify objects. Children who are able to conserve mass or volume are
aware that an object's mass or volume can remain the same even if its dimensions
change.23 For example, they are able to compare
their portion of food with a standardized shape, or estimate the amount of
beverage they consume by using a glass with dimensions that differ from the
one from which they drank. The ability to conserve mass and volume may be
only beginning to emerge in some of the girls (aged 8 to 12 years) who participated
in this research. Therefore, some girls in this age range may not be cognitively
ready to accurately estimate portion sizes. We designed the manipulative props
approach to overcome some of these difficulties. However, even these methods
did not improve children's food portion estimates compared with the standard
2-dimensional visuals.
Valid estimates of dietary intake are needed to inform clinical and
policy decision making. Because dietary recalls are used so frequently in
both clinical nutrition assessment and research on children's diets, additional
research on alternative methods of obtaining portion size information from
school-aged children is greatly needed. We have no reason to believe that
our results are unique to African American girls, but similar research is
also needed in boys and other ethnic and age groups. Furthermore, because
dietary recalls are typically conducted on the foods consumed on the previous
day, research that examines the accuracy of portion size estimates 24 hours
after consumption, not within minutes as in this research, is needed. In addition,
future research that examines how the type of food or children's preference
for a food influences their ability to estimate portion sizes is needed. Pending
the development of more accurate methods, our results suggest that children's
dietary recall estimates are most accurate for ranking children's relative
intakes, but caution should be used in interpreting quantitative food and
energy estimates.
| What This Study Adds
Despite widespread use of self-reported food intake data in child nutrition
research and clinical practice, there is little research that examines the
validity of these data. Our research describes the errors associated with
children's portion size estimates by means of 2 food portion measurement aids:
the standard 2-dimensional food portion visuals and manipulative props. Although
both methods were generally effective at ranking girls' relative intakes,
errors in quantitative estimates of gram weight of foods and energy intakes
were large. Children's inability to accurately estimate the food quantities
they consumed limits the usefulness of their self-reported food intake for
developing food and nutrient intake recommendations for children. These inaccuracies
are important to acknowledge when making clinical decisions and policy recommendations
regarding children's nutrition.
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AUTHOR INFORMATION
Accepted for publication April 12, 2002.
This study was supported by grant UO1 HL62663 from the National Heart,
Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.
We thank Helena C. Kraemer, PhD, for statistical advice; Ann Varady,
MS, for statistical programming; and Jennifer Styles, RD, Rebecca Brown, Sarah
Green, Ayisha Owens, Nikko Quartey, MS, and Deanne Samuels, PhD, for assisting
with data collection.
Corresponding author and reprints: Donna M. Matheson, PhD, Stanford
Center for Research in Disease Prevention, Stanford University School of Medicine,
1000 Welch Rd, Palo Alto, CA 94304-1825 (e-mail: matheson{at}stanford.edu).
From the Stanford Center for Research in Disease Prevention, Department
of Medicine (Drs Matheson and Robinson and Mss Hanson and McDonald), and Division
of General Pediatrics, Department of Pediatrics (Dr Robinson), Stanford University
School of Medicine, Palo Alto, Calif.
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