Lecture 10 tests for proportions
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LECTURE 10 TESTS FOR PROPORTIONS. EPSY 640 Texas A&M University. TESTS FOR PROPORTIONS. Proportions involve Nominal scale data Univariate case 1 sample k samples Bivariate case association agreement. 1 Sample case. Proportion = k/n where k= # in category of interest, n=sample size

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Lecture 10 tests for proportions
LECTURE 10TESTS FOR PROPORTIONS

EPSY 640

Texas A&M University


Tests for proportions
TESTS FOR PROPORTIONS

  • Proportions involve Nominal scale data

  • Univariate case

    • 1 sample

    • k samples

  • Bivariate case

    • association

    • agreement


1 sample case
1 Sample case

  • Proportion = k/n where k= # in category of interest, n=sample size

  • Var (p) = p(1-p)/n = pq/n, where p= proportion, n= # in sample, and q=1-p

  • If you have a single case, then Var(p)=p(1-p) = pq

  • ex. Single roll of two dice, getting a 7 has p=1/6, so for a single roll, Var(p)=1/6*5/6 = 5/36, SD=sqrt(5/36) = .373


1 sample p confidence interval
1 Sample p Confidence Interval

  • The confidence interval due to Ghosh is

  • p  [n/(n+z2){ p + z2/2n  z pq/n + z2/rn2}

  • ex. The probability of admission to med school is .08 for applicants. The 95% confidence interval around a single case is (.01, .81)

  • For the roll of a 7, the 95% CI is (.005, .846)



Testing k independent proportions
Testing k independent proportions proportion

  • For k categories with pi = proportion in category i and p1+p2+...pk=1 , we can test 1. All proportions are the same 2. Each pi = i based on theory or previous data


Testing k independent proportions1
Testing k independent proportions proportion

  • For k categories with pi = proportion in category i and p1+p2+...pk=1

  • All proportions are the same p.=1/k:

    X2 = N[(p1 - p.)2/p.+ (p2-p.)2/p.+...(pk-p.)2/p.]

    with degrees of freedom k-1

    ex. The reported proportions of ethnic groups in Bryan is .33 Caucasian, .42 Hispanic, and .25 African American with 8000 students. The chi square is 347.2 with 2df. This is significant at p=.0001


Testing k independent theoretical proportions
Testing k independent theoretical proportions proportion

  • For k categories with pi = proportion in category i and p1+p2+...pk=1, each proportion tested against a theoretical value i

    X2 = N[(p1 - 1)2/ 1+(p2- 2)2/ 2+...(pk- k)2/k]

    with degrees of freedom k-1

    ex. The reported proportions of ethnic groups in Bryan is .33 Caucasian, .42 Hispanic, and .25 African American with 8000 students. National proportions are .74, .14, and .12.

    The chi square is 7423.9 with 2df. This is significant at p=.0001


Bivariate association
Bivariate Association proportion

  • Two nominal variables are measured for N persons (eg. Gender and aggression status)

  • An R x C table of proportions is computed prc = proportion for row r, column c

  • Then, the sum of a row of proportions is pr. and for a column the sum is p.c

  • Association is defined as departure from average expected proportion for each cell cumulated over the cells


Chi square association
Chi Square Association proportion

  • For an R x C table of proportions with the sum of all cell proportions equal to 1.0,

    X2 = N[(p11 - p1.p.1)2/ p1.p.1 +(p12- p1.p.2 )2/ p1.p.2 +...(prc- pr.p.c )2/ pr.p.c]

    with (R-1)(C-1) degrees of freedom


Chi square association1
Chi Square Association proportion

  • For an R x C table of proportions with the sum of all cell proportions equal to 1.0,

    X2 = SUM [ (Oij – Eij ]2 / Eij

    Eij = expected cell count based on row and column averages, = N*p.j*pi.

16

12

22

50

11

23

10

44

27 35 32


OBSERVED CELLS proportion

DEM REP IND

M

F

X2(2,.05)=5.99


Chi square distribution
Chi Square Distribution proportion

X2 = zi2, where zi= z-score for score I


TAAS PASS % proportion

ETHNIC %

TAAS PASS N

TAAS PASS PROPORTIONS

Question: Is school passing rate related to ethnicity?


TAAS % PASSING proportion

N=840

Expected %

(TAAS% - Expected)2 /Expected

Chi Square statistic

significant p< .001

Conclusion: school is associate with ethnic TAAS pass rate


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