MAKING IT USEABLE    Weighting       What Weights to Use When Merging Files

There are numerous analyses of NPTS data that require merging two or more files. These are most commonly done when:

1. The data for one of the variables is on another file, or
2. Rates are being computed and the denominator must use information from a file different than that used in calculating the numerator.

Elaborating on point A, when a table uses variables from more than one file, determine what variable is in the CELLS of the table. Use the weight that goes with the variable in the cells. For example, to create a table "Main Means of Work Transportation by Sex for Workers with Bus Available to Their Household" you would need to recognize that:

• The data on gender and the main means to work is on the Person File, but
• Bus availability to the households is in the Household Files, and
• The table cells are number of workers, which is a person characteristic and thus, the Person Weight is used for the table.

This is shown in the table below.

Table 4
Main Means of Work Transportation by
Sex for Workers Where Bus is Available                            
Availability Exists

The creation of the above table is discussed in detail as Example 6 in Module 5 - Analysis.

Discussing point B, the most difficult situation involves tables that contain rates. In calculating trips per person, the trip file and the person file must be merged because there are persons who participated in the survey but did not make any trips on their assigned travel day. These people show up on the person file, but not on the trip file. (See WHEN IS A RECORD ON THE FILE, Module 3 - Data Files). When computing a rate, such as daily person trips per worker, the analyst must:

• Step 1
  Weight the person trips for workers using the travel day trip weight (WTTRDFIN)

Table 5
Travel Day Person Trips by Worker Status

Worker status of respondent	Travel Day Person Trips (Annualized)
	Sample Size	Sum	Percent
Yes	245,870	228,393,909,745	60.3
No	163,155	150,536,453,591	39.7
All	409,025	378,930,363,336	100.0

• Step 2
  Weight the workers using the person weight (WTPERFIN)

Table 6
Number of Persons by Worker Status

Worker status of respondent	Number of Persons
	Sample Size	Sum	Percent
Yes	51,928	131,697,367	54.5
No	43,432	109,977,633	45.5
All	95,360	241,675,000	100.0

and

• Step 3
  Divide person trips From Step 1 by workers from Step 2.
  
  

OR

228,393,909,745 trips per year by workers = 1734 trips/year
              131,697,367 workers

which is 4.75 trips per day per worker.

The above steps are discussed in detail as Exercise 7 in Module 5 - Analysis.

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