When should you use planned comparisons?

When should you use planned comparisons?

In the context of one-way ANOVA, the term planned comparison is used when:

• You focus in on a few scientifically sensible comparisons rather than every possible comparison.
• The choice of which comparisons to make was part of the experimental design.

What is a planned comparison in statistics?

The term planned comparison is used when you focus in on a few scientifically sensible comparisons. You don’t do every possible comparison. And you don’t decide which comparisons to do after looking at the data. Instead, you decide — as part of the experimental design — to only make a few comparisons.

What planned contrasts?

Planned contrasts typically involve the comparison of just two means. The approach is to develop a set of weights that eliminate any group means that are not involved in the comparison by giving them a zero weight and to specify the group means to be compared by giving them opposite values, usually -1 and +1.

What is a planned comparison test?

Planned comparison tests require that you focus in on a few scientifically sensible comparisons. You can’t decide which comparisons to do after looking at the data. The choice must be based on the scientific questions you are asking, and be chosen when you design the experiment. Hence the term planned comparisons.

What is the difference between planned comparisons and post hoc tests?

A planned comparison is something you are committing to before you see your data, and will run no matter what the results look like. A post-hoc comparison is more opportunistic. You look at that because, when you looked at the data, that particular comparison looked interesting.

What is the difference between planned contrasts and post hoc comparisons?

They aren’t really the same. A planned comparison is something you are committing to before you see your data, and will run no matter what the results look like. A post-hoc comparison is more opportunistic. You look at that because, when you looked at the data, that particular comparison looked interesting.

Why is ANOVA more powerful than T test?

Why not compare groups with multiple t-tests? Every time you conduct a t-test there is a chance that you will make a Type I error. An ANOVA controls for these errors so that the Type I error remains at 5% and you can be more confident that any statistically significant result you find is not just running lots of tests.

What do planned contrasts do?

Can you do planned comparisons instead of ANOVA?

You either did planned comparisons instead of an overall F for ANOVA, OR you used a post-hoc test and compared all the cell means, picking out the significant ones. Now it seems that these are two ends of a spectrum, where the degree of ‘planning’ varies.

How are planned comparisons and post hoc tests different?

Planned Comparisons. Planned comparisons and post hoc tests used to be separate entities. You either did planned comparisons instead of an overall F for ANOVA, OR you used a post-hoc test and compared all the cell means, picking out the significant ones. Now it seems that these are two ends of a spectrum, where the degree of ‘planning’ varies.

How many groups are there in a comparison?

Note that the first contrast compares groups 1 and 2 vs. groups 3 and 4. The second and third comparisons contrast 1 vs. 2 and 3 vs. 4, so that all of the comparisons are independent. We have 4 categories, so we can have at most 3 independent contrasts.