Data from Independent Groups: Categorical Measures Flashcards
can only take on one of a limited number of values, often simply yes or no.
- Categorical or Nominal data
very rarely used as an appropriate measure of central tendency.
mode
- It does not tell much.
Summarizing Categorical Data
First way is to look at the data is to use the ____
Absolute Difference
Three main ways of showing the difference between two proportions. (fourth one as well)
* All look very similar to each other and it is often not clear which one people are talking about.
Absolute and Relative Change
We usually describe the % of people in each group and the differences between them.
We also get on to the 95% CIs.
Summary Statistics
the percentage of the decrease achieved by the group receiving intervention compared with the group that did not receive the intervention (i.e., the control group).
Relative Risk Decrease.
Third way of showing the difference two proportions. *
Trickier than percentages and proportions but most common way.
odds ratio
One more method of presenting the effect of an intervention which is commonly used in medicine, though less commonly used in psychology.
- Also known as NNH or number needed to harm.
Number Needed to treat (NNT)
If we know the probability, p, we can calculate the odds using the following formula:
____
- Odds = p / 1 – p
Although it is possible to compute for all the descriptive statistics that we calculated, most of them are rarely used so the only one we are going to concentrate on is the odds ratio.
Confidence Intervals
The NNT is very easy to calculate. It is simply * NNT = ____
1 / Absolute Risk Difference
Developed by Pearson and sometimes known as the Pearson χ² test.
Chi-Square χ²
we have to calculate a statistic called ___, (Greek letter nu, pronounced “new” or “noo”)
v
is a bit like the standard error of the odds ratio.
v
There is only one way to calculate the probability value given the plethora of ways of displaying the difference between two proportions. (Sort of only one way)
* The test is called
Chi-square test.
The first stage in the χ² test is to put the values into a table, but ____to it.
add totals
We have to calculate the expected values for each cell, which are referred to as ___
E
The E are the values that we would expect if the null hypothesis were ___, the null hypothesis in this case being that the task type had no effect.
true
The expected values are given by:
E = R x C / T
Where ____ refers to the total for a given row, ___ the total for a given column, and ___ for grand total.
R
C
T
All we need to know is the distance between the ___ value and the _____ value so we can take the differences and add them up. (Almost but not quite)
observed
expected
The difference needs to take account of the ___
sample size
An assumption made by the χ² test is that all of the expected values (in a 2 x 2 table) must be greater than __
5
If the difference between the observed and expected values is 6, and the expected value is also 6, we are out by a____
* On the other hand, if the expected value is 1,000,000, then that is ___
long way
pretty close.
If the table is larger than 2 x 2, then___ of the expected values need to be above 5.
80%
2nd, when we have a 2 x 2 table the χ² test is a little bit liberal, and statisticians do not like ____
liberal tests.
One approach in dealing with this is to use ______ or _______
Yates’ Correction for Continuity
Continuity Correction.
The values contained within the bars means “take the absolute value”, which means if there is a ______ ignore it.
minus sign
The problem with Yates’ correction is that it makes the test a little conservative so the _____ rate is now smaller than 0.05.
Type I Error
In fact, the correction only matters when the sample size is relatively ____
So if the sample size is large, the correction makes little difference.
small.
If the sample size is small, there is a better test we can use called the _____
Fisher’s Exact test.
Fisher’s exact test gives the probability of getting the exact result that was found in the study.
- However, we are not just interested in the exact result, we are interested in any result that is more____than the result we have
extreme
the Idea of _____ is that for some events we can work out the exact probability of them occurring without needing to use test statistics and tables
Fisher’s exact test
With Fisher’s exact test there is _____, and no need to look anything up in a table.
no test statistic
