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   3-E. Survey Response Rates		      

The 1995 NPTS data were collected during the period from May 1995 through July 1996.  There were several stages of data collection.  First, a sample of telephone numbers was screened to identify residential households.  Second, an adult member of the household was asked a series of questions about the persons and vehicles of the household.  Following this household interview, the household was assigned a travel day for trip reporting.  Then, travel diaries for each person 5 years and older were prepared and mailed to the household.  Following the household's travel day, interviewers called to conduct the person interview for each eligible household member.  During the person interviews, travel diary information was recorded in the computer, along with responses to a number of additional questionnaire items. A summary of the overall response rates, as well as the rates obtained at each stage of the survey process are documented in this section. 

SUMMARY OF RESPONSE RATES The 1995 NPTS response rates are summarized in Table 3-1, which includes the partial response rate experienced at each stage of the survey, and the overall response rate up to that point in the process.  The table shows that 73.2 percent of the in-scope sample numbers completed the screening process. Household interviews were completed for 75.6 percent of the completed screening cases, or 55.3 percent of all in-scope sample cases.  Over 93 percent of the completed household interview cases accepted the travel diaries, and sufficient person-level interviews were completed for 72.1 percent of these households to classify them as "useable" for the 1995 NPTS.  Within the useable households, person level interviews were completed with 92.2 percent of the eligible persons. Table 3-1 shows that the overall response rates were 55.3 percent for household level data and 34.3 percent for person level data. 

Table 3-1
Summary of Overall Response Rates
 

Rate

Rate

Calculation

Telephone Number Screening

73.2

73.2%

------

Household Interview Rate

75.6

55.3%

.732 x .756

Diary Acceptance Rate

93.3

51.6%

.553 x .933

Useable Household Rate

72.1

37.2%

.516 x .721

Person Interview Rate

92.2

34.3%

.372 x .922


Another way of viewing the survey response rates, is with the actual numbers of sample cases, as follows: 
112,960 - telephone numbers in-scope   
82,663 - households completing screening 
58,276 - households accepting diary 
42,033 - useable households, that contained: 
103,466 - persons eligible 
95,360 - persons interviewed. 

TELEPHONE NUMBER SCREENING

Table 3-2 shows the results of telephone calls to screen the 160,048 sample telephone  numbers. 

Table 3-2
Telephone Number Screening Response Data

Out-of-Scope-Total

Number

Percent

43,882

27.4%

Non-working number

15,393

9.6%

Beeper/pager

2,089

1.3%

Mobile phone

953

0.6%

Modem/fax

4,193

2.6%

Other nonresidential

1,204

0.8%

Business

19,270

12.0%

Group Quarters

483

0.3%

Determined later

297

0.2%


In-Scope-Total

Number

Percent

105,269

65.8%

Completed Screening

82,663

51.6%

Answering Machine

4,938

3.1%

Refusal

12,233

7.6%

Language Barrier

1,315

0.8%

Other non-interview

2,393

1.5%

Trials exhausted

1,727

1.1%


Eligibility Unknown

Number

Percent

10,897

6.8%

Total Sample Cases

Number

Percent

160,048

100.0%



SCREENING RESPONSE RATE The telephone number screening response rate calculation is illustrated in Table 3-3.  The total of in-scope numbers was estimated by adding a portion of the numbers whose eligibility status was unknown to the number determined to be in-scope.  More specifically, the 70.58 percent rate of in-scope numbers was applied to the 10,897 numbers whose scope could not be determined, which yielded 7,691 numbers that were presumed to be in scope.  These were added to the 105,269 in-scope numbers, for an estimated total in-scope of 112,960 numbers.  Of this total, 82,663 numbers, or 73.2 percent, completed eligible screening.


Table 3-3
Screening Response Rate Calculation
 

Number

Total Sample Cases

160,048

Telephone Number Screening:  
Out-of-Scope Numbers

43,882

In-Scope Numbers

105,269

Scope Determined

149,151

Percent In-Scope

70.58%

Scope not Determined

10,897

Presumed In-Scope

7,691

Estimated Total In-Scope

112,960

Completed Eligible Screenings

82,663

Screening Response Rate

73.2%

HOUSEHOLD INTERVIEW RATES

The interviewers attempted to complete both the household screening and the household interview on a single call whenever possible.  Toward the end of the household interview, the respondents were told the travel day selected for the household, and they were asked to complete the travel diaries they would be receiving in the mail in a few days.  They were also told that a monetary incentive of $2.00 per eligible person would be sent along with the diaries, as a token of appreciation for the time it takes to complete them.

As Table 3-4 shows, over 19 percent of the 82,663 households identified in the telephone number screening process refused to provide the household interview information.  In total, household interviews were completed with 62,468 household respondents, or 75.6 %. In 4,192 of these, the household respondent either refused to verify their mailing address, if we knew it before the interview, or refused to provide the mailing address, if we didn't know it in advance.  These cases are shown in Table 3-4 as completing the household interview, but refusing to accept the travel diaries.

Table 3-4
Household Interview Response Data
Household Interviews

Number

Percent

Completed - diary accepted

58,276

70.5%

Completed - diary refused

4,192

5.1%

Completed - total

62,468

75.6%

Refusal

16,039

19.4%

Language Barrier

704

0.8%

Other non-interview

888

1.1%

Trials exhausted

2,564

3.1%

Total

82,663

100.0%


PERSON INTERVIEW RATES

At the completion of the household interview, the household's travel day was assigned 12 to 18 days in the future. This allowed time to prepare and mail the diaries, and for the mail to be delivered to the household shortly before their travel day. Following the travel day, interviewers called to retrieve the diary information and complete the person interview for each eligible household member. Table 3-5 shows that there were 146,317 eligible persons in the 58,276 households that completed the household interview and accepted the diary. Of these 146,317 people, person interviews were completed with 97,881 people or 66.9 percent. An additional 5.1 percent were refusals and 14.6 percent were for persons that could not be contacted despite repeated attempts during the six-day interviewing period. Table 3-5 also shows the breakdown of completed interviews by whether they were completed by the persons themselves or by proxy respondents. Note that the 1995 NPTS required proxy interviews for all eligibles 5 to 13 years of age; it allowed proxy interviews for eligibles who were 14 years and older.

USEABLE HOUSEHOLD RATE

The 1995 NPTS defined a useable household as one in which person interviews were completed with at least 50 percent of the household's eligible adults. Table 3-6 shows that 42,033, or 72.1 percent, of the 58,276 households that accepted the travel diaries met this requirement. Person interviews were completed for all eligible persons in the majority of the useable households. The 1995 NPTS data files contain the information collected from these 42,033 useable households.

PERSONS IN USEABLE HOUSEHOLDS

Table 3-7 shows the person response rate information within 1995 NPTS useable households. Data for each of the 95,360 responding persons in useable households is included in the 1995 NPTS data files, and accounts for nearly all of the 97,881 (see Table 3-5) person interviews completed in the 1995 NPTS survey. Note that the proxy interviews include persons age 5 through 13 where the interview must be by proxy, and 14 through 17 year- olds who have a high incidence of proxy interviews.