Whenever we are going to predict a population parameter using sample data, it is always affected by some error. And this error is known as “Margin of Error”.
So the Interval Estimation can be expressed as:
Confidence Interval is dependent on:
The 95% confidence level is the middle portion in the above figure and the leftover section on the left and right side is 5% combined which is 2.5% on each side.
Hence when we are referring to the z-distribution table, we need to look at the value 95 + 2.5 = 97.5 to get the critical value of 95% confidence level.
Look at the row and column value for the cell 0.975.
Row value is 1.9
Column value is 0.06
Hence Critical value will be 1.9 + 0.06 = 1.96 to get 95% confidence level
Problem:
We are given with the Standard Deviation of population weight to be 4 Kgs, and a sample of 100 people is chosen and their average weight is 62 kgs. What is the Standard Error, Margin of Error and Confidence interval for Confidence Level 95%?
Let us write what is given:
Standard Deviation of population = 4
Sample Size = 100
Sample Mean = 62
Confidence Level = 95
Standard Error = 4 / Square_root(100) = 4 / 10 = 0.4
For 95% confidence level, critical value is 1.96
Margin of Error = 1.96 x Standard Error
Margin of Error = 0.784
Confidence Interval = Sample mean +- Margin of Error
Lower bound of Confidence Interval = 62 - 0.784 = 61.216
Upper bound of Confidence Interval = 62 + 0.784 = 62.784
Hence the confidence interval will be [61.216, 62.784] with 95% of confidence level.
This means that there is 95% probability that the estimated confidence interval [61.216, 62.784] will contain the true population mean.
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