# Shared Flashcard Set

## Details

Political Statistics
Statistics in political science, such as polling
17
Political Studies
02/27/2011

Term
 variance
Definition
 subtract values from mean, square all the differences, add up all the differences, then divide by the number of observations
Term
 standard deviation
Definition
 take sq root of variance. (subtract values from mean, square em, add em up, divide by # observations, take sq root)
Term
 steps for measures of assoc w/ 2 ordinal variables (K's gamma, Tau-b, Tau-c, Somer's d)
Definition
 create frequency table w/ cells for totals. id the pairs which are concordant, discordant, tied on the row, tied on the column. For each type of pair (concordant, discordant, TR, TC), multiply the 2 #s together and add up all the products. For ex. (4 x 3) + (4 x 6)...
Term
 how to calc covariance
Definition
 subtract values from mean, multiply differences for one variable by the the differences for the other variable, add products up, divide by # of observations (not the total number of the two variables added up, but just the number of pairs observations being compared)
Term
 how to calc std error for cont variables
Definition
 subtract mean from each of values, square em, add up squares, divide by # of observations minus 1, take sq root, then divide by sq root of the number of observations
Term
 two-tailed vs one-tailed hypothesis
Definition
 if population mean could be above or below the test value, it's two-tailed. if the pop mean can only be above or only be below the test value, it's one-tailed
Term
 when to use the different means tests -- Z-test and paired T-test
Definition
 use Z-test when trying to find out if a population mean is equal to a particular value. Paired t-test is used to test whether the mean of one variable is equal to the mean of another variable
Term
 steps for Z-test
Definition
 specify the hypothesis that the population mean is/isnt equal to (the test value). Write down the alternative hypothesis and whether it's one-tailed or two-tailed. Calc mean if it's a cont variable. then calc std error. Then plug into formula and take absolute value of answer -- that's the z-score. look up z-score on table and find p-value. if it's a two-tailed hypothesis, multiply p-value by 2.If p<.05, reject the null with 95% certaintyIf p<.01, reject the null with 99% certainty
Term
 steps for paired t-test
Definition
 1. Subtract the second variable from the first variable, create a column of differences2. Calculate the mean of the column of differences3. Calculate the standard error of the column of differences4. Calculate degrees of freedom = (number of non-missing observations – 1 )5. Calculate T = ( mean / standard error ) 6. Use the T-score table. Find the row that matches up to the degrees of freedom.Look up on table. If your T-value is BIGGER than the value for t.025, for example, the means are significantly different at the .025 level for a one-tailed test, and .05 for a two tailed test. If smaller, there's minimal difference between the two means
Term
 steps for chi-square test and when to use it
Definition
 -use it to test the amt of correlation between two categorical variables -create cross tab. add 'total' column and row. calc expected freqs, plug into chi-square formula for each cell, add up those -- that's the chi-square value. calc degrees of freedom and look up in table.   If your chisquare value is BIGGER than the value for .025, the association is significant at the .025 level
Term
 percentile
Definition
 -order observations smallest to largest-Divide the percentile N by 100, and multiply number of observations. If this number has a decimal, round up- Starting from the smallest value, count up as many observations as the number derived in step 3. The value in that observation is the Nth percentile***if the number from step three is a whole number before rounding, count up from smallest observation that number and that number plus 1. Add the two and divide by 2 to get percentile
Term
 median
Definition
 -order observations smallest to largest-multiply # of observations times 0.5-if decimal round up-Starting from the smallest value, count up as many observations as the number derived in step 3. The value in that observation is the Nth percentile***if the number from step three is a whole number before rounding, count up from smallest observation that number and that number plus 1. Add the two and divide by 2 to get percentile
Term
 std error for binary variables
Definition
 use 2nd formula under "Margin of Error" on formula sheet (one w/ "p" value)
Term
 how to calc margin of error
Definition
 -count number who gave each answer (Y) and (N)-compute the proportion who gave the first answer using this formula: p= ___Y_____ Y + N-calc std error using the p-value calc' in previous step-plug into this formula: p + or - 1.96 *SE for 95% confidence (replace 1.96 with 2.58 for 99% confidence)
Term
 N and m in Kendall's tau-c
Definition
 N is the number of observations in the table (not the number of cells). add up all the numbers in the cells to get it.m is the either the number or rows or columns – whichever is smaller.
Term
 How they fool you with stats
Definition
 Non-random samples, Improper generalization (Republicans won more seats in the last election, but that does not necessarily mean voters want more restrictive abortion law), Small samples / No standard deviations (margins of error) reported, Self-selected respondents, Loaded questions:
Term
 Convert odds ratio to percent
Definition
 subtract one and multiply by 100
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