Sampling Error for the NPTS Full Sample

MAKING IT USEABLE
Interpreting Estimates
Sampling Error for the NPTS Full Sample

For the NPTS sample, with its stratified sample design, sampling errors for the entire U.S. are on the order of one to one-half of one percent of the mean values, meaning that the sample values have a very high probability of being very close to the values that would have been found from measurement of the entire population, instead of a sample.

Another way of looking at this is to say that the sample is very efficient, because by measuring only 42,000 households, we have obtained estimates of population measures that are extremely close to those that would have been obtained by measuring all 99,000,000 households. The following table provides examples of sampling error in the NPTS.

Table 8
Sampling Error in the NPTS

What was estimated

The Estimated Value

The Standard Deviation

The Number of Observations

The Sampling Error

The average household size 2.584 persons 1.409 persons 42,033 households 0.0095 persons

The average number of vehicles owned by a household 1.779 vehicles 1.036 vehicles 42,033 households 0.0068 vehicles

The average work trips per household per day 2.79 work trips per household per day 3.710 work trips per household per day 42,033 households 0.025 work trips per household per day

The average number of workers per household 1.299 workers 0.959 workers 42,033 households 0.0064 workers

The average annual vehicle miles of travel per vehicle 12,226 miles per vehicle per year 12,286 miles per vehicle per year 65,628 vehicles 66.25 miles per vehicle per year

Bicycle trips per household per day 0.082 bicycle trips per household 0.635 bicycle trips per day (i.e. about eight times the mean) 42,033 households 0.0043 bicycle trips per household

The example shows that sampling errors for the entire U.S. are very small in comparison to the mean values. However, this statement applies to the entire sample and for estimates for the entire country; it does not necessarily hold if one looks at only a portion of the country.

What was estimated	The Estimated Value	The Standard Deviation	The Number of Observations	The Sampling Error
The average household size	2.584 persons	1.409 persons	42,033 households	0.0095 persons
The average number of vehicles owned by a household	1.779 vehicles	1.036 vehicles	42,033 households	0.0068 vehicles
The average work trips per household per day	2.79 work trips per household per day	3.710 work trips per household per day	42,033 households	0.025 work trips per household per day
The average number of workers per household	1.299 workers	0.959 workers	42,033 households	0.0064 workers
The average annual vehicle miles of travel per vehicle	12,226 miles per vehicle per year	12,286 miles per vehicle per year	65,628 vehicles	66.25 miles per vehicle per year
Bicycle trips per household per day	0.082 bicycle trips per household	0.635 bicycle trips per day (i.e. about eight times the mean)	42,033 households	0.0043 bicycle trips per household