# I do not know how to calculate my sample size…

#### Maria voltaren

##### New Member
Hello,

can someone help me calculate my sample size and showing me the steps of how it should be done?

I am going to do a study to see a linkage between genotypic resistance and suppression of HIV.

When taking the some HIV drug, some people are able to suppress the HIV virus and some people are not.

I want to see if among the group of people who are able to suppress the virus, the percentage of people who have genotypic resistant virus is significantly less than among the group of people who are not able to suppress

e.g., 10% of people who suppress have genotypic resistant virus vs 60% of people who do not suppress have genotypic resistant virus.

Note: Even people who have genotypic resistant virus can suppress, at least for a short while.

I want to see this for two groups of patients:
• a group of patients who are on medication for 1 year and
• a group of patients who are on medication for 5 years.
• I want to see if there are differences between these two groups

e.g. 10% of people who suppress have genotypic resistant virus and 70% of people who do not suppress have genotypic resistant virus in patients who are on medication for 1 year versus 20% of people who suppress have genotypic resistant virus and 80% of people who do not suppress have genotypic resistant virus in patients who are on medication for 5 years).

Data I know:
• It is estimated that 15% of all patients have genotypic resistance (I assume this for both patient groups; who are on treatment for 1 year or for 5 years).

• I do not know the % of patients who suppress. That is something I want to find out.

• My population is composed of 2359 patients.
o 986 patients are on medication for 1 year and
o 1045 patients are on medication for 5 years

Assuming a 95% confidence level

How do I calculate my sample size? Do I have to have two samples, 1 for patients on medication for 1 year and another for patients on medication for 2 years or just 1 sample that includes both groups of patients?

Thank very much for any help that you can give me.

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