Asked Nov 21, 2019

A political candidate has asked you to conduct a poll to determine what percentage of people support her.

If the candidate only wants a 3% margin of error at a 99% confidence level, what size of sample is needed?



Expert Answer

Step 1

Here, there is no information about the population parameter. Hence, use the conservative estimate π=0.5.

Step-by-step procedure to obtain the value of z using MINITAB:

  • Choose Graph > Probability Distribution Plot choose View Probability > OK.
  • From Distribution, choose ‘Normal’ distribution.
  • Click the Shaded Area
  • Choose Probability and Both Tail for the region of the curve to shade.
  • Enter the Probability value as 01.
  • Click OK.

Output using the MINITAB software is given below:


Image Transcriptionclose

Distribution Plot Normal, Mean-0, StDev=1 0.4 0.3 0.2 0.1 0.005 0.0 0.005 2.576 -2.576 0 X Density

Step 2

From the output, the value of z is 2.576 for 99% confidence...


Image Transcriptionclose

(1-) N =i E 2.576 0.5(1-0.5) 0.03 1,843.3 1,844


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