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SMSA. Data for the SMSA's in Survey Group Il are currently available in all forms. Data for the SMSA's in Survey Group Ill are presently only available in Current Population Reports, Series P-23, No. 72, "Selected Characteristics of Travel to Work in 20 Metropolitan Areas: 1976." No data for the SMSA's in Survey Group I have yet been released.

Data from the 1975 National Travel-to-Work Supplement are currently available in Current Population Reports, Series P-23, No. 99, "The Journey to Work in the United States:

1975" and in the form of unpublished tables. As in the SM SA samples, the unpublished National tables cross-classify commuters and characteristics of the commuting trip by the socioeconomic characteristics obtainable from the Annual Housing Survey, which include age, sex, race, household relationship, and earnings. Information concerning these unpublished data may be obtained by writing to the Chief, Population Division, U.S. Bureau of the Census, Washington, D.C. 20233.

Table 1. Place of Residence, by Place of Work, for the Miami SMSA
(Workers 14 years old and over. Numbers in thousands. SMSA as of the 1970 census)

Reported a fixed place of work

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[100.0]

100.0 Miami city

(100.0) 100.0 Outside central city.

[100.0

100.0 Note:

Percents in brackets, [ ], are of all workers.

98.7 97.4

51.5 23.8

47.2 73.6

1.3 2.6

[9.2] G. .3] [9.9

[...] (0.5)

Table 2. Principal Means of Transportation to Work, by Place of Residence, for the Miami SMSA

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Table 3. Principal Means of Transportation to Work, by Selected Characteristics of Commuters,

for the Miami SMSA (Workers 14 years old and over. SMSA as of the 1970 census. For explanation of symbols, see text)

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SNSA

Table 4. Principal Means of Transportation, by Distance to Work, for the Miami SMSA
(Workers 14 years old and over. SMSA as of the 1970 census. For explanation of symbols, see text)

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Table 5. Principal Means of Transportation, by Travel Time to Work, for the Miami SMSA
(Workers 14 years old and over. SMSA as of the 1970 census. For explanation of symbols, see text)

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AHS-SMSA Housing Inventory Estimation Procedure

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The DOT Travel-to-Work Supplement data are based on interviews completed during the period April 1975 through March 1976 in 21 SMSA's as part of the enumeration for the Year 11 Annual Housing Survey (AHS) sponsored by the Department of Housing and Urban Development. Under the sponsorship of the Department of Transportation (DOT), the 1975 AHS-SMSA questionnaire included a supplementary group of questions pertaining to travel-to-work. In the four largest SMSA's, the survey sample consisted of about 15,000 housing units, and for the remaining 17 SMSA's, the survey was based on a sample of about 5,000 housing units.

In this SMSA, 4,537 housing units were eligible for interview in AHS. Of these sample units, 216 interviews were not obtained because, for occupied sample units, the occupants were not at home after repeated visits or were unavailable for some other reason; or for vacant units, no informed respondent could be found after repeated visits. In addition to units eligible for interview, 473 units were visited but found not to be eligible for interview because they were condemned, unfit, demolished, converted to group quarters use, etc. Within the interviewed households of this SMSA there were 8,248 persons 14 years and older. Of these, 21 persons did not respond to the DOT Travelto-Work Supplement.

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Selection of the AHS-SMSA sample. The sample for the SMSA's which are 100 percent permit-issuing was selected from two sample frames-units enumerated in the 1970 Census of Population and Housing in areas under the jurisdiction of permit-issuing offices (the permit-issuing universe) and units constructed in permit-issuing areas since the 1970 census (the new construction universe). In addition, the sample for those SMSA's which are not 100 percent permit-issuing included a sample selected from a third frame-those units located in areas not under the jurisdiction of permit-issuing offices (the nonpermit universe). This SMSA is 100 percent permit-issuing. A more detailed description of the selection of the sample can be found in the AHS Series H-170 reports for 1975.

DOT Supplement Adjustments. For the DOT Supplement, the weight resulting from the AHS-SMSA estimation procedure described above was adjusted to account for persons in households that were interviewed for AHS-SMSA who did not respond to the travel-to-work section of the question. naire. This noninterview adjustment factor was calculated separately for each sector of each SMSA. Within each sector of each SMSA, a noninterview factor was computed separately for sex, age, and marital status categories.

The final adjustment for persons interviewed for the DOT Supplement was an additional ratio estimation procedure. This procedure was designed to adjust the AHS-SMSA sample estimate of persons 14 years and older in each SMSA to an independently derived current estimate of that same population group. In SMSA's where there was no evidence of differential undercoverage of persons within the sectors, the sample estimate of persons 14+ in the SMSA was adjusted to an independently derived estimate of persons 14+ in the SMSA. For SMSA's where there was evidence of differential

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ESTIMATION

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The estimation procedure for the DOT Travel-to-Work Supplement utilized the AHS-SMSA housing inventory esti

undercoverage within the sector, this ratio estimation was performed separately by central city and balance of the SMSA. The factor used for the ratio estimation procedure was calculated as follows:

Independent estimate of persons 14+ in the SMSA (or sector)

Sample estimate of persons 14+ in the SMSA (or sector)

Reinterview program. No reinterview program was undertaken for the DOT Travel-to-Work Supplement. However, for the 1975 AHS-SMSA sample a study was conducted to obtain a measurement of some of the components of the nonsampling error associated with the AHS estimates. Results of this study may be a useful indicator of the accuracy to be expected in the travel-to-work data which was collected as a supplement to the AHS-SMSA Jata. A detailej description can be found in the AHS Series H-170 reports for 1975.

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The numerator of this ratio was based on the Census Bureau's estimates of population 14+ as of October 1, 1975. The denominator of this ratio was obtained from the weighted estimate of persons interviewed for the DOT Supplement, using the existing weight after the DOT Supplement noninterview adjustment had been applied. For this SMSA, a person ratio estimate factor was calculated for each sector.

The weight that resulted from the application of this final adjustment was the tabulation weight utilized to produce final tabulations.

The effect of this person ratio estimation, as well as the overall estimation procedure, was to reduce the sampling error for most statistics below what would have been obtained by simply weighting the results of the sample by the inverse of the probability of selection. Since the population 14 years and older of the sample differed somewhat by chance from the actual population in each city, SMSA balance, or SMSA as a whole, it can be expected that the sample estimates will be improved when the sample population is brought into agreement with known independent estimates of the actual population. RELIABILITY OF THE DATA

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Coverage errors. With respect to errors of coverage and estimation for missing data, it is believed that the AHS new construction sample had deficiencies with regard to the representation of both conventional new construction and new mobile homes (and trailers) in permit-issuing areas. Although it is not known exactly, an estimated 17,100 conventional new construction units and 5,300 new mobile homes in permit-issuing areas in this SMSA were missed by the 1975 AHS-SMSA survey. It is felt that deficiencies also exist in non-permit-issuing areas. The 1975 AHS sample has been estimated to miss as much as 2 percent of all housing units in these areas.

Therefore, all persons 14 years or older who live in the above "missing" housing units or who live in enumerated housing units but were not detected by the enumerators had no chance for enumeration in the DOT Travel-to-Work Supplement. The person ratio estimation corrects for these deficiencies with respect to the count of persons 14+ in each SMSA. However, biases associated with estimates of travelto-work characteristics of these people may still remain. Rounding errors. With respect to errors associated with processing, the rounding of estimates introduces another source of error in the data, the severity of which depends on the statistic being measured. The effect of rounding is significant relative to the sampling error only for small percentages and medians derived from relatively large bases (e.g., median number of workers per household or median distance traveled to work).

This means that confidence intervals formed from the standard errors given may be distorted, and this should be taken into account when considering the results of the survey. Sampling errors. The particular sample used for this survey is one of a large number of possible samples of the same size that could have been selected using the same sample design. Even if the same schedules, instructions, and enumerators were used, estimates from each of the different samples would differ from each other. The variability between estimates from all possible samples is defined as the sampling error. One common measure of this sampling error is the standard error which measures the precision with which an estimate from a sample approximates the average result of all possible samples.

In addition, the standard error as calculated for this survey also partially measures the variation in the estimates due to some nonsampling errors, but it does not measure, as such, any systematic biases in the data. Therefore, the accuracy of the estimates depends on both the sampling and

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The DOT Travel-to-Work Supplement. One possible source of bias in the DOT Travel-to-Work Supplement data is proxy interviewing. That is, responses for a particular worker may have been given by someone else who is not as knowledgeable as the worker himself. For example, the person available for the interview may not know how long it takes the reference person (worker) to travel to work or whether or not the principal means of transportation to work is satisfactory to the worker. Although it is known that biases due to proxy interviewing, as well as other nonsampling errors, could exist in the DOT Travel-to-Work Supplement, their magnitude is unknown.

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