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How to Find a Confidence Interval: The Easy Way!

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how to find a confidence interval

How to Find a Confidence Interval: Overview

When we talk about confidence intervals, we’re dealing with data. For example, let’s say the manager for that job you applied for told you he would get back with you in a “couple of days.” A couple of days could mean two. Or three. Or there might be a paperwork backlog and it could be a week. It definitely doesn’t mean in an hour. So your confidence interval would probably be between 2 and 4 days.

Perhaps the trickiest part of confidence intervals is recognizing the various parts needed for the formula, like z a/2. This article breaks everything down into simple steps and shows you how to find the most common confidence intervals for populations.

How to Find a Confidence Interval: Steps

Sample question: 510 people applied to the Bachelor’s in Elementary Education program at Florida State College. Of those applicants, 57 were men. Find the 90% confidence interval of the true proportion of men who applied to the program.

Step 1: Read the question carefully and figure out the following variables:

  • α : subtract the given confidence interval from 1.
    1-.9=.10
  • z α/2: divide α by 2, then look up that area in the z-table. Not sure how to read a z table? See this article, area to the right of a z-score for an explanation on how to read the z-table.
    .1/2=.0500. The closest z-value to an area of .0500 is 0.13
  • phat: Divide the proportion given (i.e. the smaller number)by the sample size.
    57/510=0.112
  • qhat : Subtract phatfrom 1.
    1-0.112 = 0.888

Step 2: Multiply phatby qhat.
0.112 x 0.888 = 0.099456

Step 3: Divide step 2 by the sample size.
0.099456 / 510 = 0.000195011765

Step 4:Take the square root of step 3:
sqrt(0.000195011765) = 0.0139646613

Step 5: Multiply step 4 by z a/2:
0.0139646613 x 0.13 = 0.0182

Step 6:: For the lower percentage, subtract step 5 from phat.  
0.112-0.0182 =  0.0938 = 9.38%

Step 7:For the upper percentage, add step 5 to phat.
0.112 + 0.0182 =  0.1302 = 13.02%

That’s how to find a confidence interval!

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16 thoughts on “How to Find a Confidence Interval: The Easy Way!

  1. Angie Widdows

    Wow. I didn’t realize there were so many steps involved. This example helped alot because it broke everything up, step by step. Was helpful to have the link to the Z-table.

  2. Lisa Barcomb

    Yeah these problems have a lot of steps to them but once you get them down they are really easy because you have the Z table. And in order for you to do your problems you have to know how to use the tables. Which in a way I thought they were fun.

  3. Jennifer Foster

    z α/2: divide the given confidence interval by 2, then look up that area in the z-table.
    .9/2=.4500. The closest z-value to an area of .4500 is 1.65

    I don’t understand this….can someone please explain?
    What am I not getting?

  4. Stephanie

    Hi Jennifer,
    Why don’t you send me your working out, and I’ll tell you where you are going wrong (or at least, point out which specific part you don’t understand…is it how to look up a number in the z-table? Or something else?)
    Stephanie

  5. Jennifer Foster

    The closest z-value to an area of .4500 is 1.65 …

    This is where I am really stuck. How did you get that? I can look up .45 on the table and it does not come out to be 1.65

  6. Stephanie

    Jennifer,
    What do you get when you look up .4500 in the center of the table? Let me know and I should be able to see where you are going wrong.
    Sometimes it helps to work backwards…look up 1.65 in the table (i.e. 1.6 in the left hand column and .05 in the top row). Do you see the .4505 where it intersects?
    Stephanie

  7. Donna Allen

    Once again, I found this explanation very helpful. It was easier to follow your step by step instructions than the guided solution in mathzone.

  8. April Fulton

    I have had the same question how do you get 1.65. But from this step by step and the teacher explaining to another student the same question, I see I am not the only one with these questions. This site is a big help!

  9. Myrlande Blanc

    I was wondering how to get confidence interval with standard deviation 0.3, confidence level of 90%, and satisfaction score of 4.5

  10. Stephanie

    Hi, Myrlande,
    Can you post your question on the forum? Unfortunately, I don’t have the time to answer math questions here.
    Thanks,
    Stephanie

  11. Bill

    I have two z-tables in my text , for 1.65 it’s .9505 and for -1.65 it’s .0495, how does your table have .4505?