Compare proportions for two or more groups in the data
The compare proportions test is used to evaluate if the frequency of occurrence of some event, behavior, intention, etc. differs across groups. The null hypothesis for the difference in proportions across groups in the population is set to zero. We test this hypothesis using sample data.
We can perform either a one-tailed test (i.e.,
greater than) or a two-tailed test (see the
Alternative hypothesis dropdown). A one-tailed test is
useful if we want to evaluate if the available sample data suggest that,
for example, the proportion of dropped calls is larger (or smaller) for
one wireless provider compared to others.
We will use a sample from a dataset that describes the survival status of individual passengers on the Titanic. The principal source for data about Titanic passengers is the Encyclopedia Titanic. One of the original sources is Eaton & Haas (1994) Titanic: Triumph and Tragedy, Patrick Stephens Ltd, which includes a passenger list created by many researchers and edited by Michael A. Findlay. Lets focus on two variables in the database:
Suppose we want to test if the proportion of people that survived the
sinking of the Titanic differs across passenger classes. To test this
hypothesis we select
pclass as the grouping variable and
calculate proportions of
Choose level) for
Variable (select one)).
Choose combinations box select all available
entries to conduct pair-wise comparisons across the three passenger
class levels. Note that removing all entries will automatically select
all combinations. Unless we have an explicit hypothesis for the
direction of the effect we should use a two-sided test (i.e.,
two.sided). Our first alternative hypothesis would be ‘The
proportion of survivors among 1st class passengers was different
compared to 2nd class passengers’.
The first two blocks of output show basic information about the test (e.g.,. selected variables and confidence levels) and summary statistics (e.g., proportions, standard error, margin or error, etc. per group). The final block of output shows the following:
Null hyp. is the null hypothesis and
Alt. hyp. the alternative hypothesis
diff is the difference between the sample proportion
for two groups (e.g., 0.635 - 0.441 = 0.194). If the null hypothesis is
true we expect this difference to be small (i.e., close to zero)
p.value is the probability of finding a value as
extreme or more extreme than
diff if the null hypothesis is
If we check
Show additional statistics the following
output is added:
Pairwise proportion comparisons Data : titanic Variables : pclass, survived Level : Yes in survived Confidence: 0.95 Adjustment: None pclass Yes No p n n_missing sd se me 1st 179 103 0.635 282 0 8.086 0.029 0.056 2nd 115 146 0.441 261 0 8.021 0.031 0.060 3rd 131 369 0.262 500 0 9.832 0.020 0.039 Null hyp. Alt. hyp. diff p.value chisq.value df 2.5% 97.5% 1st = 2nd 1st not equal to 2nd 0.194 < .001 20.576 1 0.112 0.277 *** 1st = 3rd 1st not equal to 3rd 0.373 < .001 104.704 1 0.305 0.441 *** 2nd = 3rd 2nd not equal to 3rd 0.179 < .001 25.008 1 0.107 0.250 *** Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
chisq.value is the chi-squared statistic associated
diff that we can compare to a chi-squared
distribution. For additional discussion on how this metric is calculated
see the help file in Basics > Tables > Cross-tabs. For
each combination the equivalent of a 2X2 cross-tab is calculated.
df is the degrees of freedom associated with each
statistical test (1).
2.5% 97.5% show the 95% confidence interval around the
difference in sample proportions. These numbers provide a range within
which the true population difference is likely to fall
There are three approaches we can use to evaluate the null hypothesis. We will choose a significance level of 0.05.1 Of course, each approach will lead to the same conclusion.
Because the p.values are smaller than the significance level for each pair-wise comparison we can reject the null hypothesis that the proportions are equal based on the available sample of data. The results suggest that 1st class passengers were more likely to survive the sinking than either 2nd or 3rd class passengers. In turn, the 2nd class passengers were more likely to survive than those in 3rd class.
Because zero is not contained in any of the confidence intervals we reject the null hypothesis for each evaluated combination of passenger class levels.
Because the calculated chi-squared values (20.576, 104.704, and
25.008) are larger than the corresponding
critical chi-squared value we reject the null hypothesis for
each evaluated combination of passenger class levels. We can obtain the
critical chi-squared value by using the probability calculator in the
Basics menu. Using the test for 1st versus 2nd class passengers
as an example, we find that for a chi-squared distribution with 1 degree
of freedom (see
df) and a confidence level of 0.95 the
critical chi-squared value is 3.841.
In addition to the numerical output provided in the Summary
tab we can also investigate the association between
survived visually (see the Plot tab). The
screen shot below shows two bar charts. The first chart has confidence
interval (black) and standard error (blue) bars for the proportion of
yes entries for
survived in the sample.
Consistent with the results shown in the Summary tab there are
clear differences in the survival rate across passenger classes. The
Dodge chart shows the proportions of
survived side-by-side for each passenger
class. While 1st class passengers had a higher proportion of
no the opposite holds for the 3rd
prop.test function to compare
proportions. When one or more expected values are small (e.g., 5 or
less) the p.value for this test is calculated using simulation methods.
When this occurs it is recommended to rerun the test using Basics
> Tables > Cross-tabs and evaluate if some cells may have an
expected value below 1.
Less than or
Greater than) critical values must be obtained by using the
normal distribution in the probability calculator and squaring the
The more comparisons we evaluate the more likely we are to find a “significant” result just by chance even if the null hypothesis is true. If we conduct 100 tests and set our significance level at 0.05 (or 5%) we can expect to find 5 p.values smaller than or equal to 0.05 even if there are no associations in the population.
Bonferroni adjustment ensures the p.values are scaled appropriately given the number of tests conducted. This XKCD cartoon expresses the need for this type of adjustments very clearly.
This is a comparison of proportions test of the null hypothesis that the true population difference in proportions is equal to 0. Using a significance level of 0.05, we reject the null hypothesis for each pair of passengers classes evaluated, and conclude that the true population difference in proportions is not equal to 0.
The p.value for the test of differences in the survival proportion for 1st versus 2nd class passengers is < .001. This is the probability of observing a sample difference in proportions that is as or more extreme than the sample difference in proportions from the data if the null hypothesis is true. In this case, it is the probability of observing a sample difference in proportions that is less than -0.194 or greater than 0.194 if the true population difference in proportions is 0.
The 95% confidence interval is 0.112 to 0.277. If repeated samples were taken and the 95% confidence interval computed for each one, the true difference in population proportions would fall inside the confidence interval in 95% of the samples
1 The significance level, often denoted by \(\alpha\), is the highest probability you are willing to accept of rejecting the null hypothesis when it is actually true. A commonly used significance level is 0.05 (or 5%)
Add code to
> Rmd to (re)create the analysis by clicking the
icon on the bottom
left of your screen or by pressing
ALT-enter on your
If a plot was created it can be customized using
plot(result, plots = "bar", custom = TRUE) + labs(title = "Compare proportions")).
> Visualize for details.
For an overview of related R-functions used by Radiant to evaluate proportions see Basics > Proportions.
The key function from the
stats package used in the
compare_props tool is
Copy-and-paste the full command below into the RStudio console (i.e., the bottom-left window) and press return to gain access to all materials used in the hypothesis testing module of the Radiant Tutorial Series: