# Forum post (Confidence Interval)

Confidence Intervals for the Mean sigma known

Main Post

Assuming .05 as your alpha value, make up a problem similar to this one. Make up 36 gas prices for your example. Use price values in your list that some you made up and be sure they are similar to the prices you have had in your area. List the values in your post.

Instructions for your response. You will respond to at least 2 classmates.

In your response, describe what you did and give the mean, standard deviation and confidence interval.

Example below:

• First find the mean. List all 36 gas prices in cells A1 to A36 in your excel spreadsheet.
• On a different cell type in =average(A1:A36) and the hit enter to find the average
• On a different cell type in =stdev.s(A1:A36) or =stdevA(A1:A36) and enter to find the standard deviation
• Then type this in an excel cell =confidence(.05,.43,36) and hit enter. This is the E value
• So take the mean of 2.65 and you have (2.65-E, 2.65+E) for your confidence interval

Formula you used above for the confidence interval:

The formula is E= zsubc * sigma / sqrt n

Where the left hand endpoint is xbar – E and the right hand endpoint is xbar + E

Just a review of your instructions:

• First you will make up a problem with alpha = .05 and give the values for the problem. This is your main post.
• Then will make 2 responses explaining all work and giving the solutions to 2 classmates problems.

Instructions: Your initial post should be at least 250 words. Please respond to at least 2 other students. Responses should be a minimum of 100 words and include direct questions.

To earn full credit for this forum, you must respond to at least two (2) of your peers and post your initial post by Thurs.