HD: Food Security (2011)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To estimate the effects of the Special Supplemental Nutrition Program for Women, Infants and Children (WIC) participation on birth weight.

Inclusion Criteria:
  • Participation by mother in National Longitudinal Survey of Youth in 1996 or earlier
  • Birth of a child between 1990 and 1996
  • Valid information on prenatal WIC participation.
Exclusion Criteria:
  • Mother not participant of NLSY Survey in 1996 or earlier years
  • Invalid information regarding prenatal WIC participation
  • Child born before 1990 or after 1996.
Description of Study Protocol:

Recruitment

  • Data drawn from 1996 and earlier survey waves of the NLSY
  • Recruitment methods for the NLSY not discussed
     

Design

  • Data was drawn from the NLSY surveys completed prior to 1996
  • Evaluated WIC participation and its impact on mother reported birth weights of all children from NLSY study participants and sibling samples
  • Comparison of WIC participation status and birth weight among siblings


Intervention

  • Prenatal WIC participation

 

Statistical Analysis

  • Ordinary least squares model used as general model for the relationship between WIC participation and early child outcomes
  • Huber-White robust standard errors to account for the lack of independence of observations based on siblings born to the same mother
  • Fixed effect model used to remove bias from time-invariant maternal and family components
  • Multivariate analyses
  • Sibling sample size sufficient size to detect a 7% impact on birth weight with 90% power. 

 

Data Collection Summary:

Timing of Measurements

  • Initial interviews for the NLSY completed in 1979
  • NLSY obtained maternal reports on whether WIC benefits had been received in the preceding calendar year prior to giving birth as well as birth weight starting in 1990

Dependent Variables

  • Birth weight measured by maternal report

Independent Variables

  • Prenatal WIC participation determined by maternal report
     

Control Variables

  • Prenatal food stamp participation
  • Total income
  • Alcohol consumption during pregnancy
  • Smoking during pregnancy
  • Armed Forces qualifying test score
  • Self-esteem score
  • Deviant behavior score
  • Prenatal urban residence
  • Prenatal maternal education
  • Parents' education level
  • Number of siblings
  • Ethnicity
  • Gender of child
  • Gestational length.
     

 

Description of Actual Data Sample:

Initial N

1,984 NLSY children born between 1990 and 1996 (50% male).

Attrition (Final N)

  • Sibling sample, consisting of NLSY children who were born between 1990 and 1996 and had at least one sibling born in the same time period, was comprised of 969 children
  • Discordant sibling groups, where siblings differed in terms of their mother's participation in the WIC program before their birth, totaled 71.

Age

Mothers were 25 to 38 years of age at time of birth.

Ethnicity

Based on total sample

  • Black: 11±31%
  • Hispanic: 6±24%.

Other Relevant Demographics

  • Total income (in 1,000s): $47.82±26.67
  • Prenatal urban residence: 82±38%
  • Prenatal maternal education: 13.75±2.45
  • Parents' education level: 12.06±2.91.

Anthropometrics

Birth weight discussed in Summary of Results section.

Location

United States.

 

Summary of Results:

Key Findings

  • Estimates based on simple OLS models implied that participation in WIC was associated with a statistically-insignificant 0.025 increase in logged birth weight
  • Controls for a more extensive set of maternal and child characteristics more than doubled the OLS-based estimated impact to a statistically-significant 0.055
  • Sibling fixed-effects models suggest a positive impact of prenatal WIC participation on birth weight
  • After the full set of prenatal maternal variables had been taken into account, prenatal WIC participation was associated with a 0.075-unit difference in siblings' logged birth weight: Based on the 88-ounce low birth weight cut-off, this difference translates to an estimated impact of 6.6 ounces.

Variables

Analysis 1 OLS
(Prenatal WIC participation and child demographics only)

Analysis 1 Fixed Effects
(Includes full set of variables)

Analysis 2 OLS
(Prenatal WIC participation and child demographics only)

Analysis 2 Fixed Effects
(Includes full set of variables)

Prenatal WIC Participation

0.025±0.016

0.034±0.034

0.055±0.02
P<0.01

0.075±0.042
P<0.10

Other Findings

  • More complete OLS model also suggested that prenatal maternal smoking and drinking were associated with significant decrements in birth weight
  • Higher maternal cognitive skills were significantly linked to higher birth weights
  • Significant negative association between prenatal receipt of food stamps and birth weight
  • Both race and sex differences in birth weight were noted
  • First-born children were more likely to be lower in birth weight than higher birth order children.
Author Conclusion:

The study's approach was to use sibling fixed-effects modeling techniques to remove potentially biasing effects of unobserved family-specific characteristics. This approach found the positive effects of prenatal WIC participation on infant birth weight persist when the potential for bias is minimized.

Funding Source:
Government: US Department of Agriculture/Institute for Research on Poverty Food Assistance Small Grants Program
Reviewer Comments:
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? No
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) Yes
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? No
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? Yes
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? N/A
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes