Question

Castaneda v. Partida is an important court case in which statistical methods were used as part of a legal argument. When reviewing this case, the Supreme Court used the phrase "two or three standard deviations" as a criterion for statistical significance. This Supreme Court review has served as the basis for many subsequent applications of statistical methods in legal settings. (The two or three standard deviations referred to by the Court are values of the z statistic and correspond to P-values of approximately 0.05 and 0.0026.) In Castaneda the plaintiffs alleged that the method for selecting juries in a county in Texas was biased against Mexican Americans. For the period of time at issue, there were 180,650 persons eligible for jury duty, of whom 142,875 were Mexican Americans. Of the 892 people selected for jury duty, 338 were Mexican Americans.

(a) What proportion of eligible voters were Mexican Americans? Let this value be p_o. (Round your answer to four decimal places.)

(b) Let p be the probability that a randomly selected juror is a Mexican American. The null hypothesis to be tested is H_o: p = p_o. Find the value of p̂ for this problem, compute the z statistic, and find the P-value. What do you conclude? (A finding of statistical significance in this circumstance does not constitute a proof of discrimination. It can be used, however, to establish a prima facie case. The burden of proof then shifts to the defense.) (Use α = 0.01. Round your test statistic to two decimal places and your P-value to four decimal places.)

Z=

P-value=

(c) We can reformulate this exercise as a two-sample problem. Here we wish to compare the proportion of Mexican Americans among those selected as jurors with the proportion of Mexican Americans among those not selected as jurors. Let p₁ be the probability that a randomly selected juror is a Mexican American, and let p₂ be the probability that a randomly selected nonjuror is a Mexican American. Find the z statistic and its P-value. (Use α = 0.01. Round your test statistic to two decimal places and your P-value to four decimal places.)

Z=

P-value=

Answer 1

(a) p0 = 142875/180650 = 0.79

(b)

n= 892

= -30.15

p value is less than 0.0001

the result is highly significant

We reject the null hypothesis

there is significant evidence to conclude that proportion of Mexican American selected is different from proportion of eligible Mexican American voters.

(c)

where

P = 0.79, Q=0.21  ,n1= 892,n2=179758

z= -30.20

p value is less than 0.0001

we reject the null hypothesis

There is sufficient evidence to conclude that proportion of MA in selected jury is different from proportion of MA in non jury