Consider a population variable measured in square-feet. The population standard deviation is 15 square-feet. How many observations do we need in our sample in order to be able estimate a 95% confidence interval with only 2.5 square-feet for error margin?

Given Information:  

standard deviation = σ = 15 ft²

Confidence interval = 95%

Margin of error = 2.5 ft²

Required Information:  

Sample size = n = ?

Sample size = n ≈ 139

Step-by-step explanation:  

The required number of observations can be found using ,

Me = z(σ/√n)

Where Me is the margin of error, z is the corresponding z-score of 95% confidence interval, σ is the standard deviation and n is the required sample size.

Rearrange the above equation to find the required number of sample size

√n = σz/Me

n = (σz/Me)²

For 95% confidence level, z-score = 1.96

n = (15*1.96/2.5)²

n = 138.29

since the sample size can't be in fraction so,

n ≈ 139

Therefore, a sample size of 139 would be needed.