## Survey Methodology3. Response Rates

This section describes the computation of the response rates for the 2002 National Transportation Availability and Use Survey. The overall response rate is the product of the screener interview response rate and the extended interview response rates. The screener and extended interviews were computed in accordance with the standards defined by the Council of American Survey Research Organizations (CASRO, 1982). The response rate was also computed using the survival method (Brick, Montaquila, and Scheuren, 2002) for reference. The weights for the survey data files were adjusted using the CASRO definition as described in section 4.

To compute the screener response rate, the study classified each screener disposition code as residential, nonresidential or undetermined, as Table 3.1 shows. To classify these cases, a variable, RESDFLG (residential flag), was created with values of YES (residential), NO (nonresidential) and UNK (undetermined or unknown). Table 3.1 shows the classification of the screener disposition codes and the number of cases for the survey. Notice that the only screener disposition codes that are considered undetermined are NA and NM (ring no answer, and answering machine). This three-part breakdown of cases for the RESDFLG variable is presented in Table 3.2.

The number of completed screener interviews (cases coded as CA, CB, CD, and CS) is 10,327, as defined in the first four rows in Table 3.1.

The screener response rate (unweighted) is computed as

Different definitions exist for the screener response rate. These definitions depend on the way the number of residential cases among the undetermined is estimated.

### 3.1 Computing a CASRO Response Rate

In CASRO, the proportion of undetermined numbers that are residential numbers is estimated using the observed residential and nonresidential cases in the sample. This proportion is computed as

Thus, CASRO assumes that 40.18 percent of the undetermined cases (NA and NM) are residential.

Using the proportion based on the CASRO formula, the screener response rate is then

.

The extended response rate (unweighted) is computed as

.

Table 3.3 shows the number of completed cases by extended interview disposition code. A total of 5,019 out of 5,755 extended interviews were completed.

Therefore, the extended response rate is

and the overall CASRO response rate is

OVERALLRR = SCRRR * EXTRR = 0.642492 * 0.872111 = 0.560324 = 56.03%.

### 3.2Computing a Survival Analysis Response Rate

The survival analysis method is used to compute response rates when there is an additional effort made to resolve the residential status of the cases coded as NA and NM (for example, by making repeated calls to a sample of these cases to determine their status). If there is no such effort, the survival method will produce the same results as the CASRO formula (Brick, Montaquila, and Scheuren, 2002). If there is an additional effort, the screener response rate is usually higher than the CASRO rate.

This additional effort was done for another RDD study, the very large National Survey of American Families (NSAF), an RDD survey with approximately 46,000 completed interviews. This allowed the computation of a response rate for the Transportation Availability and Use Study based on the NSAF survival method. NSAF was very similar to this survey in the following ways:

• A national survey;
• Conducted during the same time; and
• The proportion of observed residential cases to total cases (cases determined to be residential/total cases) was almost identical.

In NSAF, an additional effort was made to resolve the residential status of the NA and NM cases by making repeated calls to a sample of these unknowns. Therefore, in NSAF the response rate and residential rate were computed using the survival method. The observed residential rate is a population value; therefore it should be the same for both this and the NSAF surveys. For this reason, one can apply the response rate computed in NSAF to this survey.

The study team proceeded as follows when applying the NSAF rate to this survey:

1. Using the NSAF residency rate, and the formula for the residential rate, the study computed the estimated proportion of NA and NM cases that were residential.
2. The study team then used the proportion of NA and NM cases that were considered residential from (1) in the screener response rate formula to compute the screener response rate.

The proportion of NA and NM cases that were residential is computed by solving for p in the formula for the residential rate. This formula for the residential rate is

The NSAF residency rate was computed as 0.391, so solving for p and substituting the number of cases, the value of p is

.

Using this p, in the screener response rate formula, the screener response rate based on the survival analysis is

.

The overall response rate is then calculated as

.

That is an increase of 1.55% compared to the CASRO rate. Table 3.4 summarizes the response rates. The weights for the survey data files were computed using the CASRO definition.

### References

Council of American Survey Research Organizations. (1982). On the definition of response rates (special report of the CASRO task force on completion rates, Lester R. Frankel, Chair). Port Jefferson, NY: CASRO.

Brick, J.M., Montaquila, J., and Scheuren, F.J. (2002). Estimating residency rates for undetermined telephone numbers, Public Opinion Quarterly, Vol. 66, 18-39.