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## Nominal Data

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**Nominal Data**Greg C Elvers**Parametric Statistics**• The inferential statistics that we have discussed, such as t and ANOVA, are parametric statistics • A parametric statistic is a statistic that makes certain assumptions about how the data are distributed • Typically, they assume that the data are distributed normally**Nonparametric Statistics**• Nonparametric statistics do not make assumptions about the underlying distribution of the data • Thus, nonparametric statistics are useful when the data are not normally distributed • Because nominally scaled variables cannot be normally distributed, nonparametric statistics should be used with them**Parametric vs Nonparametric Tests**• When you have a choice, you should use parametric statistics because they have greater statistical power than the corresponding nonparametric tests • That is, parametric statistics are more likely to correctly reject H0 than nonparametric statistics**Binomial Test**• The binomial test is a type of nonparametric statistic • The binomial test is used when the DV is nominal, and it has only two categories or classes • It is used to answer the question: • In a sample, is the proportion of observations in one category different than a given proportion?**Binomial Test**• A researcher wants to know if the proportion of ailurophiles in a group of 20 librarians is greater than that found in the general population, .40 • There are 9 ailurophiles in the group of 20 librarians**Binomial Test**• Write H0 and H1: • H0: P £ .40 • H1: P > .40 • Is the hypothesis one-tailed or two-tailed? • Directional, one-tailed • Determine the statistical test • The librarians can either be or not be ailurophiles, thus we have a dichotomous, nominally scaled variable • Use the binomial test**Binomial Test**• Determine the critical value from a table of critical binomial values • Find the column that corresponds to the p value (in this case .40) • Find the row that corresponds to the sample size (N = 20) and a (.05) • The critical value is 13**Binomial Test**• If the observed number of ailurophiles (9) is greater than or equal to the critical value (13), you can reject H0 • We fail to reject H0; there is insufficient evidence to conclude that the percentage of librarians who are ailurophiles is probably greater than that of the general population**Normal Approximation to the Binomial Test**• When the sample size is greater than or equal to 50, then a normal approximation (i.e. a z-test) can be used in place of the binomial test • When the product of the sample size (N), p, and 1 - p is greater than or equal to 9, then the normal approximation can be use**Normal Approximation to the Binomial Test**• The normal approximation to the binomial test is defined as: x = number of observations in the category N = sample size P = probability in question**Normal Approximation to the Binomial Test**• A researcher wants to know if the proportion of ailurophiles in a group of 100 librarians is greater than that found in the general population, .40 • There are 43 ailurophiles in the group of 100 librarians**Normal Approximation to the Binomial Test**• Write H0 and H1: • H0: P £ .40 • H1: P > .40 • Is the hypothesis one-tailed or two-tailed? • Directional, one-tailed • Determine the statistical test • The librarians can either be or not be ailurophiles, thus we have a dichotomous, nominally scaled variable • Use the z test, because n ³ 50**Normal Approximation to the Binomial Test**• Calculate the z-score**Normal Approximation to the Binomial Test**• Determine the critical value from a table of area under the normal curve • Find the z-score that corresponds to an area of .05 above the z-score • That value is 1.65 • Compare the calculated z-score to the critical z-score • If |zcalculated| ³ zcritical, then reject H0 • 0.612 < 1.65; fail to reject H0**c2 -- One Variable**• When you have nominal data that has more than two categories, the binomial test is not appropriate • The c2 (chi squared) test is appropriate in such instances • The c2 test answers the following question: • Is the observed number of items in each category different from a theoretically expected number of observations in the categories?**c2 -- One Variable**• At a recent GRE test, each of 28 students took one of 5 subject tests • Was there an equal number of test takers for each test?**c2 -- One Variable**• Write H0 and H1: • H0: S(O - E)2 = 0 • H1: S(O - E)2¹ 0 • O = observed frequencies • E = expected frequencies • Specify a • a = .05 • Calculate the c2 statistic • c2=S[(Oi-Ei)2/Ei]**c2 Calculations**• c2=S[(Oi-Ei)2/Ei] • c2=7.31+2.31+0.46+0.29+0.46=10.83 • Calculate the degrees of freedom: • df = number of groups - 1 = 5 - 1 = 4 • Determine the critical value from a table of critical c2 values • df = 4, a = .05 • Critical c2a=.05(4) =9.488**c2 Decision**• If the observed / calculated value of c2 is greater than or equal to the critical value of c2, then you can reject H0 that there is no difference between the observed and expected frequencies • Because the observed c2 = 10.83 is larger than the critical c2 =9.488, we can reject H0 that the observed and expected frequencies are the same**c2 Test of Independence**• c2 can also be used to determine if two variables are independent of each other • E.g., is being an ailurophile independent of whether you are male or female? • Write H0 and H1: • H0: SS(O - E)2 = 0 • H1: SS(O - E)2¹ 0 • Specify a • a=.05**c2 Test of Independence**• The procedure for answering such questions is virtually identical to the one variable c2 procedure, except that we have no theoretical basis for the expected frequencies • The expected frequencies are derived from the data**c2 Test of Independence**• The expected frequencies are given by the formula to the right:**c2 Test of Independence**• Calculate the observed value of c2**c2 Test of Independence**• First, determine the degrees of freedom: • df = (r - 1) * ( c - 1) • In this example, the number of rows (r) is 2, and the number of columns (c) is 2, so the degrees of freedom are (2 - 1) * (2 - 1) = 1 • Determine the critical value of c2 from a table of critical c2 values • Critical c2a=.05(1)=3.841**c2 Test of Independence**• Make the decision • If the observed /calculated value of c2 is greater than or equal to the critical value of c2, then you can reject H0 that the expected and observed frequencies are equal • If this example, the observed c2 = 3.319 is not greater than or equal to the critical c2 = 3.841, so we fail to reject H0**Requirements for the Use of c2**• Even though c2 makes no assumptions about the underlying distribution, it does make some assumptions that needs to be met prior to use • Assumption of independence • Frequencies must be used, not percentages • Sufficiently large sample size**Assumption of Independence**• Each observation must be unique; that is an individual cannot be contained in more than one category, or counted in one category more than once • When this assumption is violated, the probability of making a Type-I error is greatly enhanced**Frequencies**• The data must correspond to frequencies in the categories; percentages are not appropriate as data**Sufficient Sample Size**• Different people have different recommendation about how large the sample should be, and what the minimum expected frequency in each cell should be • Good, Grover, and Mitchell (1977) suggest that the expected frequencies can be as low as 0.33 without increasing the likelihood of making a Type-I error • Small samples reduce power