The PRAMS sample of women who have had a recent live birth is drawn from the state's birth certificate file. Each participating state samples between 1,300 and 3,400 women per year. Women from some groups are sampled at a higher rate to ensure adequate data are available in smaller but higher risk populations. Selected women are first contacted by mail. If there is no response to repeated mailings, women are contacted and interviewed by telephone. Data collection procedures and instruments are standardized to allow comparisons between states.
One strength of the PRAMS surveillance system is the standardized data collection methodology. This standardized approach allows for comparisons among states and for optimal use of the data for single-state or multistate analysis. The standardized data collection methodology is prescribed in the CDC Model Surveillance Protocol.1 Each state follows the protocol, but also has the opportunity to customize some portions of it to tailor the procedures to the needs of the state.
PRAMS combines two modes of data collection; a survey conducted by mailed questionnaire with multiple follow-up attempts, and a survey by telephone. The principles and practices of mail/telephone survey methodology used by CDC are based primarily on the research of Don Dillman.2 A key aspect of his approach is to make numerous and varied contacts with sampled mothers. Here is the sequence of contacts for PRAMS surveillance:
- Preletter. This letter introduces PRAMS to the mother and informs her that a questionnaire will soon arrive.
- Initial Mail Questionnaire Packet. This packet is sent to all sampled mothers 3 to 7 days after the preletter. Its contents are described below.
- Tickler. The tickler serves as a thank you and a reminder note. It is sent 7 to 10 days after the initial mail packet.
- Second Mail Questionnaire Packet. This packet is sent to all sampled mothers who have not yet responded 7 to 14 days after the tickler has been sent.
- Third Mail Questionnaire Packet. This third packet is sent to all remaining nonrespondents 7 to 14 days after the second questionnaire.
- Telephone Follow-up. Telephone follow-up is initiated for all mail nonrespondents 7 to 14 days after mailing the last questionnaire.
The series of mailings commences 2 to 4 months after delivery. The questionnaire contains items asking about the early postpartum period; thus, the mailings are timed to ensure that all women will be able to respond for this period. The data collection cycle from the mailing of the preletter to the close of telephone follow-up lasts approximately 60 to 95 days. Each month, a stratified sample is drawn from the current birth certificate file. For each of these monthly samples, or "batches," this sequence of contacts is attempted. To assist in tracking all aspects of data collection, a customized tracking system, PRAMTrac, was developed by the CDC and installed in each participating state. PRAMTrac is designed to assist with the scheduling of mailings and telephone calls, preparing letters, and tracking responses.
The mail packets contain several items. First is a multipurpose cover letter. This letter describes PRAMS and its purpose, explains how the mother was chosen and why, solicits the mother's cooperation, describes the procedures for filling out and returning the questionnaire, explains any incentive or reward, and provides a telephone number for additional information. This letter is modified slightly for the second and third mailings, primarily by adding an additional appeal for response. Beginning in 2004, the multipurpose cover letter was divided into two documents; an introductory letter and an informed consent information sheet. Second, the questionnaire booklet is included. The questionnaire booklet has a similar appearance for each state. It is no more than 14 pages in length, has a colorful cover designed by each state, and is slightly smaller than an 8½" by 11" sheet of paper. It contains an extra page at the end for comments from the mother. A self-addressed return envelope with postage is provided for the easy return of the questionnaire. Third, a question and answer brochure is added to provide additional information about PRAMS. It contains answers to the most frequently asked questions about the survey. It can be an important tool to convince the mother to participate. Fourth, a 3-year calendar is provided to be used as a memory aid for answering the questions. Fifth, all states have adopted the use of some type of participation incentive (sent to all sampled mothers) or reward (sent to all respondents). Examples of participation incentives and rewards include: coupons for certified birth certificates, participation in a raffle for a cash award, postage stamps, bibs, cash (a dollar bill), and magnetic picture frames.
Telephone follow-up begins after the mailing of the last questionnaire. A variety of sources of telephone numbers are used to obtain a valid number for a mother, and these vary by state. Calls to a particular number are staggered over different times of the day and different days of the week. The calling period for a batch runs 2 to 3 weeks. Up to 15 call attempts are made to a number in order to reach a mother. Often, telephone interviewers arrange call-back interviews to accommodate the mother’s schedule.
The original PRAMS questionnaire was developed in 1987. Topics addressed in the PRAMS questionnaire include barriers to and content of prenatal care, obstetric history, maternal use of alcohol and cigarettes, physical abuse, contraception, economic status, maternal stress, and early infant development and health status. Learn more about the questionnaires and download the latest PRAMS questionnaires.
Each participating state draws a stratified systematic sample of 100 to 250 new mothers every month from a frame of eligible birth certificates. Most states oversample low weight births. Many states stratify by mother's race or ethnicity as well. Annual sample sizes range from 1000 to 3400, divided among three to six strata. Typically, the annual sample is large enough for estimating statewide risk factor proportions within 3.5% at 95% confidence. Estimated proportions within strata are slightly less precise (typically, they are estimated within 5% at 95% confidence).
Mothers' responses are linked to extracted birth certificate data items for analysis. Thus the PRAMS data set also contains a wealth of demographic and medical information collected through the state's vital records system. The availability of this information for all births is the basis for drawing stratified samples and, ultimately, for generalizing results to the state's entire population of births. Its availability for all sampled women, whether they responded or not, is key to deriving nonresponse weights.
For each respondent, the initial sampling weight is the reciprocal of the sampling fraction applied to the stratum. Sampling fractions in PRAMS range from 1 in 1 (for very low birth weight strata in small states) to about 1 in 211 (for normal birth weight, nonminority strata in populous states). Corresponding sampling weights, then, would range from 1 to 211.
Nonresponse adjustment factors attempt to compensate for the tendency of women having certain characteristics (such as being unmarried or of lower education) to respond at lower rates than women without those characteristics. Where multivariate analysis shows that these characteristics affect the propensity to respond in a particular stratum, the adjustment factor is the ratio of the sample size in that category to the number of respondents in the category. If analysis shows that no characteristic distinguishes respondents from nonrespondents, the adjustment factor is the ratio of the sample size in that stratum to the number of respondents in the stratum. In the first case, each category so identified has an adjustment factor; in the second, there is a single factor for the whole stratum.
The rationale for applying nonresponse weights is the assumption that nonrespondents would have provided similar answers, on average, to respondents' answers for that stratum and adjustment category. So that cells with few respondents are not distorted by a few women's answers, small categories are collapsed until each cell contains at least 25 respondents. The magnitude of the adjustment for nonresponse depends on the response rate for a category. If 80% (or 4/5) of the women in a category respond, the nonresponse weight is 1.2 (or 5/4). Categories with lower response rates have higher nonresponse weights.
Frame omission studies are carried out to look for problems that occur during frame construction. The frame noncoverage weights are derived by comparing frame files for a year of births to the calendar year birth tape that states provided to CDC. Omitted records are usually due to late processing and are evenly scattered across the state, but sometimes they are clustered by particular hospitals or counties or even times of the year. The effect of the noncoverage weights is to bring totals estimated from sample data in line with known totals from the birth tape. In mail/telephone surveillance, the magnitude of noncoverage is small (typically from 1% to 5%), so the adjustment factor for noncoverage is not much greater than 1.
Multiplying together the sampling, nonresponse, and noncoverage components of the weight yields the analysis weight. The weight can be interpreted as the number of women like herself in the population that each respondent represents.
Analyzing PRAMS data requires software that takes into account the complex sampling designs that states employ. Such software utilizes first-order Taylor series approximations to calculate appropriate standard errors for the estimates it produces.