STEP 3: SELECT THE STATISTICAL TEST
3.3.2 Comparing Two Proportions or Percentiles
This section considers hypotheses concerning two population proportions (or two population percentiles); for example, one might use these tests to compare the proportion of children with elevated blood lead in one urban area compared with the proportion of children with elevated blood lead in another area. The population proportion is the ratio of the number of elements in a subset of the total population to the total number of elements, where the subset has some specific characteristic that the rest of the elements do not. A population percentile
represents the percentage of elements of a population having values less than some threshold value C.
Let P1 represent the true proportion for population 1, and P2 represent the true proportion of population 2. The hypotheses considered in this section are:
Case 1: H0: P1 - P2 #*0 vs. HA: P1 - P2 > *0; and Case 2: H0: P1 - P2 $*0 vs. HA: P1 - P2 < *0
where *0 is some numerical value. An equivalent null hypothesis for Case 1, written in terms of percentiles, is H0: the 100P1
th
percentile minus the 100P2 th
percentile is C or larger, the reverse applying to Case 2. Since any hypothesis about the proportion below a threshold can be converted to an equivalent hypothesis about percentiles (see Section 3.2.2), this guidance will only consider hypotheses concerning proportions.
The information required for this test includes the null and alternative hypotheses (either Case 1 or Case 2); the gray region (i.e., a value *1 > *0 for Case 1 or a value *1 < *0 for Case 2, representing the bound of the gray region); the false rejection error rate " at *0; the false
acceptance error rate $ at *1; and any additional limits on decision errors.
3.3.2.1 Two-Sample Test for Proportions
PURPOSE
The two-sample test for proportions can be used to compare two population percentiles or proportions and is based on an independent random sample of m (X1, X2, . . . , Xm) from the first population and an independent random sample size n (Y1, Y2, . . . , Yn) from the second
population.
ASSUMPTIONS AND THEIR VERIFICATION
The principal assumption is that of random sampling from the two populations.
LIMITATIONS AND ROBUSTNESS
The two-sample test for proportions is valid (robust) for any underlying distributional shape and is robust to outliers, providing they are not pure data errors.
SEQUENCE OF STEPS
Directions for a two-sample test for proportions for a simple random sample and a systematic simple random sample are given in Box 3-18; an example is provided in Box 3-19.
EPA QA/G-9 Final
QA00 Version 3 - 29 July 2000
Box 3-18: Directions for a Two-Sample Test for Proportions for Simple and Systematic Random Samples
The following describes the steps for applying the two-sample test for proportions for Case 1 (H0: P1 - P2
# 0). Modifications for Case 2 (H0: P1 - P2$ 0) are given in braces { }.
STEP 1: Given m random samples X1, X2, . . . , Xm from the first population, and n samples from the second population, Y1, Y2, . . . , Yn, let k1 be the number of points from sample 1 which exceed C, and let k2 be the number of points from sample 2 which exceed C. Calculate the sample proportions p1 = k1/m and p2 = k2/n. Then calculate the pooled proportion
p ' (k
1%k2) / (m%n).
STEP 2: Compute mp1, m(1-p1), np2, n(1-p2). If all of these values are greater than or equal to 5, continue. Otherwise, seek assistance from a statistician as analysis is complicated.
STEP 3: Calculate z ' (p
1 & p2) / p(1 & p) (1/m % 1/n).
Use Table A-1 of Appendix A to find the critical value z1-" such that 100(1-")% of the normal
distribution is below z1-". For example, if " = 0.05 then z1-" = 1.645.
If z > z1-" {z < -z 1-"}, the null hypothesis may be rejected. Go to Step 5.
If z Ý z1-" {z Û -z1-"}, there is not enough evidence to reject the null hypothesis. Therefore, the
false acceptance error rate will need to be verified. Go to Step 4.
STEP 4: If the null hypothesis (H0) was not rejected, calculate either the power of the test or the sample size necessary to achieve the false rejection and false acceptance error rates. If only one false acceptance error rate ($) has been specified at P1 - P2, it is possible to calculate the sample sizes that achieve the DQOs (assuming the proportions are equal to the values estimated from the sample) instead of calculating the power of the test. To do this, calculate
where
m( ' n( ' 2(z1&"% z1&$)2P¯(1&P¯)
(P2&P 1) 2 ¯ P ' P1% P2 2 .
and zp is the pth percentile of the standard normal distribution (Table A-1 of Appendix A). If both m and n exceed m*, the false acceptance error rate has been satisfied. If both m and n are below m*, the false acceptance error rate has not been satisfied.
If m* is between m and n, use a software package like the DEFT software (EPA, 1994) or the DataQUEST software (EPA, 1996) to calculate the power of the test, assuming that the true values for the proportions P1 and P2 are those obtained in the sample. If the estimated power is below 1-$, the false acceptance error rate has not been satisfied.
STEP 5: The results of the test could be:
1) the null hypothesis was rejected, and it seems the difference in proportions is greater than 0 {less than 0};
2) the null hypothesis was not rejected, the false acceptance error rate was satisfied, and it seems the difference in proportions is less than or equal to 0 {greater than or equal to 0}; or 3) the null hypothesis was not rejected, the false acceptance error rate was not satisfied, and it seems the difference in proportions is less than or equal to 0 {greater than or equal to 0}, but this outcome is uncertain because the sample size was probably too small.
Box 3-19: An Example of a Two-Sample Test for Proportions for Simple and Systematic Random Samples
At a hazardous waste site, investigators must determine whether an area suspected to be contaminated with dioxin needs to be remediated. The possibly contaminated area (area 1) will be compared to a reference area (area 2) to see if dioxin levels in area 1 are greater than dioxin levels in the reference area. An inexpensive surrogate probe was used to determine if each individual sample is either "contaminated," i.e., over the health standard of 1 ppb, or "clean," i.e., less than the health standard of 1 ppb. The null hypothesis will be that the proportion or contaminant levels in area 1 is less than or equal to the
proportion in area 2, or H0: P1 - P2# 0 (Case 1). The decision maker is willing to accept a false rejection decision error rate of 10% (") and a false-negative decision error rate of 5% ($) when the difference in proportions between areas exceeds 0.10. A team collected 92 readings from area 1 (of which 12 were contaminated) and 80 from area 2, the reference area, (of which 10 were contaminated).
STEP 1: The sample proportion for area 1 is p1 = 12/92 = 0.130, the sample proportion for area 2 is p2 = 10/80 = 0.125, and the pooled proportion p = (12 + 10) / (92 + 80 ) = 0.128.
STEP 2: mp1 = 12, m(1-p1) = 80, np2 = 10, n(1-p2) =70. Because these values are greater than or equal to 5, continue to step 3.
STEP 3: z ' (0.130 & 0.125) / 0.128(1 & 0.128) (1/92 % 1/80)' 0.098
Table A-1 of Appendix A was used to find the critical value z0.90 = 1.282.
Because z Ý z0.90 (0.098 Ý 1.282), there is not enough evidence to reject the null hypothesis and the false acceptance error rate will need to be verified. Go to Step 4.
STEP 4: Because the null hypothesis (H0) was not rejected, calculate the sample size necessary to achieve the false rejection and false acceptance error rates. Because only one false acceptance error rate ($ = 0.05) has been specified (at a difference of P1 - P2 = 0.1), it is possible to calculate the sample sizes that achieve the DQOs, assuming the proportions are equal to the values estimated from the sample:
m( ' n( ' 2(1.282% 1.645)20.1275 (1&0.1275) (0.1)2
' 190.6 (i.e., 191samples)
where 0.1275 ' P¯ ' 0.115 % 0.055
2
Because both m and n are less than m*, the false acceptance error rate has not been satisfied.
STEP 5: The null hypothesis was not rejected, and the false acceptance error rate was not satisfied. Therefore, it seems that there is no difference in proportions and that the contaminant concentrations of the investigated area and the reference area are probably the same. However, this outcome is uncertain because the sample sizes obtained were in all likelihood too small.
EPA QA/G-9 Final
QA00 Version 3 - 31 July 2000