For persons in housing units at the time of the 1960 Census, the sampling unit was the housing unit and all its occupants; for persons in group quarters, it was the person. On the first visit to an address, the enumerator assigned a sample key letter (A, B, C, or D) to each housing unit sequentially in the order in which he first visited the units, whether or not he completed an interview. Each enumerator was given a random key letter to start his assignment, and the order of canvassing was indicated in advance, although these instructions allowed some latitude in the order of visiting addresses. Each housing unit which was assigned the key letter "A" was designated as a sample unit and all persons enumerated in the unit were included in the sample. In every group quarters, the sample consisted of every fourth person in the order listed. The 1960 statistics in this report are based on a 5-percent sample, which is a subsample of one- fifth of the original 25-percent sample schedules. The 5-percent sample was selected on the computer, using a stratified systematic Sample Design. The strata were made up as follows: For persons in regular housing units there were 36 strata, i.e., 9 household size groups by 2 tenure groups by 2 color groups; for persons in group quarters, there were 2 strata, i.e., the 2 color groups.

Although the sampling procedure did not automatically insure an exact 5-percent sample of persons, the Sample Design was unbiased if carried through according to instructions. Generally, for large areas, the deviation from the estimated sample size was found to be quite small. Biases may have arisen, however, when the enumerator failed to follow his listing and sampling instructions exactly.

Table C compares the 1959 Income distribution of families and unrelated individuals by color of head, as presented in this report, based on the 5-percent sample with the corresponding statistics based on the 25-percent sample presented in Volume I of the 1960 Census of Population. Differences in this table reflect primarily sampling error. This is relatively small and should have little influence on the interpretation of the data.

Table C. Comparison of 25-Percent and 5-Percent Sample Data on Income in 1959 of Families and Unrelated Individuals, by color of Head, For the United States: 1960

Total money income	Total					Nonwhite
	25-percent sample	5-percent sample	Percent distribution		Ratio of 25-percent sample number to 5-percent sample number	25-percent sample	5-percent sample	Percent distribution		Ratio of 25-percent sample number to 5-percent sample number
			25-percent sample	5-percent sample				25-percent sample	5-percent sample
Families
Total	45,128,397	45,148,571	100.0	100.0	1.000	4,255,521	4,261,998	100.0	100.0	0.998
Under $1,000	2,512,668	2,516,716	5.6	5.6	0.998	654,078	650,728	15.4	15.3	1.005
$1,000 to $1,999	3,373,813	3,372,929	7.5	7.5	1.000	721,787	724,019	17.0	17.0	0.997
$2,000 to $2,999	3,763,758	3,760,736	8.3	8.3	1.001	659,358	659,281	15.5	15.5	1.000
$3,000 to $3,999	4,282,945	4,273,081	9.5	9.5	1.002	574,810	575,051	13.5	13.5	1.000
$4,000 to $4,999	4,957,534	4,958,982	11.0	11.0	1.000	485,072	481,236	11.4	11.3	1.008
$5,000 to $5,999	5,563,516	5,566,790	12.3	12.3	0.999	367,075	370,934	8.6	8.7	0.990
$6,000 to $6,999	4,826,563	4,843,589	10.7	10.7	0.996	243,760	245,051	5.7	5.7	0.995
$7,000 to $9,999	9,053,220	9,063,125	20.1	20.1	0.999	370,452	374,389	8.7	8.8	0.989
$10,000 to $14,999	4,728,309	4,722,432	10.5	10.5	1.001	145,014	146,817	3.4	3.4	0.988
$15,000 and over	2,066,071	2,070,191	4.6	4.6	0.998	34,115	34,492	0.8	0.8	0.989
Median income	$5,660	$5,663	…	…	…	$3,161	$3,169	…	…	…
Unrelated individuals
Total	13,176,614	13,170,439	100.0	100.0	1.000	1,746,729	1,738,046	100.0	100.0	1.005
Under $1,000	4,948,739	4,950,102	37.6	37.6	1.000	795,004	789,525	45.5	45.4	1.007
$1,000 to $1,999	2,748,696	2,738,438	20.9	20.8	1.004	361,438	358,610	20.7	20.6	1.008
$2,000 to $2,999	1,577,738	1,579,745	12.0	12.0	0.999	242,713	241,302	13.9	13.9	1.006
$3,000 to $3,999	1,248,595	1,252,270	9.5	9.5	0.997	160,207	161,820	9.2	9.3	0.990
$4,000 to $4,999	977,474	974,440	7.4	7.4	1.003	98,409	98,054	5.6	5.6	1.004
$5,000 to $5,999	666,659	664,958	5.1	5.0	1.003	50,075	50,125	2.9	2.9	0.999
$6,000 to $6,999	372,624	378,455	2.8	2.9	0.985	19,193	18,746	1.1	1.1	1.024
$7,000 to $9,999	401,739	397,170	3.0	3.0	1.012	14,895	14,945	0.9	0.9	0.997
$10,000 and over	234,350	234,861	1.8	1.8	0.998	4,795	4,919	0.3	0.3	0.975
Median income	$1,596	$1,597	…	…	…	$1,217	$1,222	…	…	…

The statistics based on the 5-percent sample of the 1960 Census returns are estimates that have been developed through the use of a Ratio Estimation procedure. This procedure was carried out for each of the following 44 groups of persons in each of the sample weighting areas:³

Group	Sex, color, and age	Relationship and tenure
	Male white:
1	Under 5
2	5 to 13
3	14 to 24	Head of owner household
4	14 to 24	Head of renter household
5	14 to 24	Not head of household
6-8	25 to 44	Same groups as age group 14 to 24
9-11	45 and over	Same groups as age group 14 to 24
	Male nonwhite:
12-22	Same groups as male white
	Female white:
23-33	Same groups as male white
	Female nonwhite:
34-44	Same groups as male white

The sample weighting areas were defined as those areas within a State consisting of central cities of Urbanized Areas, the remaining portion of Urbanized Areas not in central cities, urban places not in Urbanized Areas, or rural areas.⁴
For each of the 44 groups, the ratio of the complete count to the sample count of the population in the group was determined. Each specific sample person in the group was assigned an integral weight so that the sum of the weights would equal the complete count for the group. For example, if the ratio for a group was 20.1, one-tenth of the persons (selected at random) within the group were assigned a weight of 21, and the remaining nine-tenths a weight of 20. The use of such a combination of integral weights rather than a single fractional weight was adopted to avoid the complications involved in rounding in the final tables. In order to increase the reliability, where there were fewer than 275 persons in the complete count in a group, or where the resulting weight was over 80, groups were combined in a specific order to satisfy both of these two conditions.

These ratio estimates reduce the component of sampling error arising from the variation in the size of household and achieve some of the gains of stratification in the selection of the sample, with the strata being the groups for which separate ratio estimates are computed. The net effect is a reduction in the sampling error and bias of most statistics below what would be obtained by weighting the results of the 5-percent sample by a uniform factor of 20. The reduction in sampling error will be trivial for some items and substantial for others. A by-product of this estimation procedure, in general, is that estimates for this sample are generally consistent with the complete count with respect to the total population and for the subdivisions used as groups in the estimation procedure. A more complete discussion of the technical aspects of these ratio estimates will be presented in another report.

The figures from the 5-percent sample tabulations are subject to Sampling Variability, which can be estimated roughly from the standard errors shown in tables D and E below. These tables⁵ do not reflect the effect of response variance, processing variance, or bias arising in the collection, processing, and estimation steps. Estimates of the magnitude of some of these factors in the total error are being evaluated and will be published at a later date. The chances are about 2 out of 3 that the difference due to Sampling Variability between an estimate and the figure that would have been obtained from a complete count of the population is less than the standard error. The chances are about 19 out of 20 that the difference is less than twice the standard error and about 99 out of 100 that it is less than 2 ½ times the standard error. The amount by which the estimated standard error must be multiplied to obtain other odds deemed more appropriate can be found in most statistical textbooks.

Table D shows standard errors of estimated numbers up to 50,000. The relative sampling errors of larger estimated numbers are somewhat smaller than for 50,000. For estimated numbers above 50,000, however, the nonsampling errors, e.g., response errors and processing errors may have an increasingly important effect on the total error. Table E shows rough standard errors of data in the form of percentages. Linear interpolation in tables D and E will provide appreciate results that are satisfactory for most purposes.

For a discussion of the Sampling Variability of medians and means and of the method for obtaining standard errors of differences between two estimates, see 1960 Census of Population. Volume I, Characteristics of the Population, Part 1, United States Summary.

Illustration: Table 1 shows that for husband-wife families with head between 25 and 64 years of age there are a total of 66,742 three- and four-person families with 2 earners or more where the head is not an earner. Of this number 6,453 (or 9.7 percent) have family income in the range $6,000 to $6,999. Table D shows that the standard error of the 6,453 is about 313. This means that the chances are approximately two out of three that the results of a complete census would not differ by more than 313 from the estimated 6,453. It also follows that, there is only about 1 chance in 100 that a complete census would differ by as much as 783, i.e., by about 2 ½ times the number estimated from table D. Table E shows that the standard error of a 97 percent figure based on 66,742 is about 0.6 percent. This means that the chances are approximately two out of three that the results of a complete census would not differ by more than 0.6 percent from the estimated 9-7 percent. It also follows that there is only about 1 chance in 100 that a complete census would differ by as much as 1,5 percent, i.e., by about 2 ½ times the number estimated in table E.
Table D. Rough Approximation to Standard Error of Estimated Number
(Range of 2 chances out of 3)

Estimated number	Standard error
50	30
100	40
250	60
500	90
1,000	120
2,500	200
5,000	280
10,000	390
15,000	480
25,000	620
50,000	880

Table E. Rough Approximation to Standard Error of Estimated Percentage
(Range of 2 chances out of 3)

Estimated percentage	Base of percentage
	500	1,000	2,500	10,000	25,000	100,000
2 or 98	3.3	2.3	1.3	0.8	0.3	0.3
5 or 95	5.0	4.0	2.3	1.0	0.5	0.3
10 or 90	7.0	5.0	3.0	1.5	0.8	0.5
25 or 75	10.0	6.8	3.8	1.8	1.0	0.5
50	11.0	7.8	4.0	2.0	1.3	0.8