# Chapter 5 Simple Random Samples

A simple random sample of size n from a population of size N has sampling weights are equal to N/n. The Horvitz-Thompson estimator of the population sum for variable X is

$\hat{T}_X = \sum_{i=1}^n \check{X}_i = \frac{N}{n} \sum_{i=1}^n X_i$

The variance of the estimator is

$\mathrm{var} [\hat{T}_X] = \frac{N - n}{N} \times N^2 \times \frac{\mathrm{var}[X]}{n}$

The first term in the variance is the finite population correction. The third term is the vriance of the mean. The second term, $$N^2$$, scales the mean to the total.

The estimator of the population mean is just the sum divided by N.

$\hat{\mu}_X = \hat{T}_X / N$ and the variance of the mean is the unscaled population sum variance.

$\hat{\mathrm{var}}[\hat{\mu}_X] = \mathrm{var} [\hat{T}_X] / N^2$

#### Example

Data set apisrs is a simple random sample of the Academic Performance Index (API) of n = 200 of the N = 6,194 schools in California.

All survey objects require that you specify the columns identifying the clusters from largest to smallest level. In simple random designs like this, there are no clusters, and you specify just the constant id = ~1. The fpc parameter specifies the column with the finite population correction. In a simple random sample, it equals N. The fpc functions to both adjust the variance estimate, and to set the observation weights.

srs_design <- svydesign(id = ~1,  fpc = ~fpc, data = apisrs)

$$\hat{T}_X$$, $$\mathrm{var} [\hat{T}_X]$$, $$\hat{\mu}_X$$, and $$\hat{\mathrm{var}}[\hat{\mu}_X]$$ are

svytotal(~enroll, srs_design)
##          total     SE
## enroll 3621074 169520
svymean(~enroll, srs_design)
##          mean     SE
## enroll 584.61 27.368

When the sample is much smaller than the population, the finite population correction makes little difference. You can omit the fpc parameter, but then you must supply the samplying weight instead. In this case, the sampling weight is 200/6194, and is in variable pw. The total and mean are unchanged, but the variance increases a little without the correction.

srs_design2 <- svydesign(id = ~1, weights = ~pw, data = apisrs)
svytotal(~enroll, srs_design2)
##          total     SE
## enroll 3621074 172325
svymean(~enroll, srs_design2)
##          mean     SE
## enroll 584.61 27.821