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 to which the key letter "A" was assigned 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. Most of the 1960 statistics in this report are based on a subsample of one-fifth of the original 25-percent sample schedules; ⁵ some, however, are based on a V-percent sample (tables 61+ and 65). ⁶ The sub-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. 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 ratios of own children under 5 per 1,000 women, as presented in this report, based on the 5-Percent sample, with 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.

Table C. Comparisons of Numbers of Women and Ratios of own children Under 5 Years Old per 1,000 Women from 25-Percent and 5-Percent Samples, By age, color, and Marital Status, For the United States: 1960

age of woman	White								Nonwhite
	Total women		Women ever married		Children under 5 years old				Total women		Women ever married		Children under 5 years old
					Per 1,000 women		Per 1,000 women ever married						Per 1,000 women		Per 1,000 women ever married
	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample	25-percent sample	5-percent sample
15 to 19 years	5,772,421	5,770,262	927,390	927,745	94	94	585	584	816,181	817,908	132,467	132,762	141	144	868	888
20 to 24 years	4,822,377	4,817,117	3,501,515	3,490,391	826	826	1,138	1,140	697,560	689,395	450,800	444,037	861	863	1,333	1,340
25 to 29 years	4,839,982	4,833,803	4,367,223	4,363,765	1,070	1,074	1,186	1,190	697,122	703,625	587,767	592,345	1,004	1,002	1,191	1,191
30 to 34 years	5,379,640	5,372,270	5,027,100	5,023,379	726	730	777	781	731,782	727,655	661,407	657,809	748	762	827	843
35 to 39 years	5,708,902	5,733,474	5,372,979	5,396,254	416	417	442	443	709,634	705,445	656,033	652,506	498	504	539	545
40 to 44 years	5,298,273	5,279,545	4,978,718	4,959,437	171	175	182	186	619,532	618,028	579,845	578,236	249	252	266	270
45 to 49 years	4,988,493	4,993,406	4,658,767	4,666,582	38	41	41	44	565,450	566,163	532,226	533,804	84	92	89	98

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: ⁷

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 urbanizes areas, or rural areas.

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 control a potential bias in the estimates, 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 of most statistics below what would be obtained by weighting the results of the 5-percent sample by a uniform factor of twenty. The reduction in sampling error will be trivial for some items and substantial for others. A byproduct 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.

The figures from the sample tabulations are subject to Sampling Variability, which can be estimated roughly from the standard errors shown in tables D, E, and F.
These tables 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 are being published in reports in Series ER 60, Evaluation and Research Program of the U.S. Census of Population and Housing: 1960. The chances are about two out of three 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 2i 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 rough 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 approximate results that are satisfactory for most purposes.

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

Estimated number	Standard error
	5-percent sample	4-percent sample
50	30	30
100	40	40
250	60	70
500	90	100
1,000	120	130
2,500	200	220
5,000	280	310
10,000	390	430
15,000	480	530
25,000	620	680
50,000	880	970

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

Estimated percentage	Base of percentage
	1,000	2,500	10,000	25,000	100,000
5-percent sample
2 or 98	2.3	1.3	0.8	0.3	0.3
5 or 95	4.0	2.3	1.0	0.5	0.3
10 or 90	5.0	3.0	1.5	0.8	0.5
25 or 75	6.8	3.8	1.8	1.0	0.5
50	7.8	4.0	2.0	1.3	0.8
4-percent sample
2 or 98	2.5	1.4	0.9	0.3	0.3
5 or 95	4.4	2.5	1.1	0.6	0.3
10 or 90	5.5	3.3	1.6	0.9	0.6
25 or 75	7.5	4.2	2.0	1.1	0.6
50	8.6	4.4	2.2	1.4	0.9

The Sampling Variability of the number of own children under 5 years old per 1,000 women depends on the variability of the distribution on which the rate is based, the size of the sample, the Sample Design (for example, the use of households as the sampling unit) and the use of ratio estimates. Estimates of standard errors for ratios of children per 1,000 women are presented in table F. The estimates are approximations that involved a number of simplifying assumptions such as the use of regression equations. If a closer approximation to the standard error of the rate of children ever born is needed, it can be calculated using the following equation:


The use of the equation will provide a closer approximation to the standard error of a rate of children ever born than the use of table F. Table F was prepared using this formula and also a regression function relating the distribution of women with 0, 1, 2, etc., children to the total number of children ever born. In any specific case, this regression function is only an approximation.

Illustration: Table 10 shows that there were 22,466 nonwhite women 19 years old in the Northeast Region and that these women had a ratio of 336 children under 5 years old per 1,000 women. Table 10 is based on a 5-percent sample, and table F shows that for an estimate of 336 children under 5 years old per 1,000 women, based on a 5-percent sample for 22,466 women, a rough approximation to the standard error is about 20. This means that the chances are about 2 out of 3 that a complete census result would not differ by more than 20 from the estimated ratio of 336 children under 5 years old per 1,000 women. It also follows that there is only about 1 chance in 100 that a complete count would differ by as much as 50, that is, by about 2 ½ times the number estimated from table F.
For a further discussion of the Sampling Variability and of the method for obtaining standard errors of differences between two estimates, see Volume I, Characteristics of the Population.

Table F. Standard Errors of Fertility Ratios, For 5-Percent and 4-Percent Samples

Sample size and number of women	Number of own children under 5 per 1,000 women
	25	50	100	200	300	400	600	800	1,000
5-percent sample
1,000	25	35	51	74	89	99	120	129	137
2,500	16	22	32	47	56	63	76	82	87
10,000	8	11	16	24	28	31	38	41	43
25,000	5	7	10	15	18	20	24	26	27
100,000	2	3	5	7	9	10	12	13	14
500,000	1	2	2	3	4	4	5	6	6
1,000,000	1	1	2	2	3	3	4	4	4
4-percent sample
1,000	27	39	57	83	99	110	133	144	153
2,500	17	24	36	52	63	70	84	91	97
10,000	9	12	18	26	31	35	42	46	48
25,000	5	8	11	17	20	22	27	29	31
100,000	3	4	6	8	10	11	13	14	15
500,000	1	2	3	4	4	5	6	6	7
1,000,000	1	1	2	3	3	3	4	5	5