# Chi-Square: Is effect A larger than effect B?

#### grey

##### New Member
Dear All,

I would like to test hypotheses like the following example: Effect A is weaker than effect B.

For Effects A and B, I have a 2 (error: present vs. absent) x 2 (group: control vs. experimental) contingency table, thus getting chi and e.g. a phi-coefficient.

My question is: How can I find out whether the resulting effect A is stronger than B?

I'll be grateful for any help!

#### MikeD

##### New Member
Hi Grey,

Look into logistic regression. I think most people would agree that a larger Odds Ratio (OR>3), typically represents a stronger association.

Take care, Mike

#### hlsmith

##### Less is more. Stay pure. Stay poor.
If you just have a single dependent variable (present/absent) and a single independent variable (control/experiment), you do not necessarily need to use logistic regression. You can just do the calculations based on your 2x2 contingency table.

Given your study design (prospective [incidence] or retrospective[prevalence]) you can either calculate relative risk or odds ratio (respectively). The important follow-up action would be also calculating the confidence interval and ensuring it does not include "1" or no difference in the comparison (ratio).

The magnitude of the effect is context and situation specific. In some situations a 1% greater relative risk could be very important, while in others perhaps you want a 300% or greater relative risk as noted in post #2.