Statistics Tip of the Week: The Margin of Error is 1/2 the width of a 2-sided Confidence Interval

In the July 18, 2017 Statistics Tip of the Week, we showed how a Confidence Interval is calculated. As illustrated in the concept flow diagram below,

• the tester selects a value for Alpha, the Significance Level.
• This, plus a Test Statistics Distribution, gives us 2 Critical Values for a 2-sided test.
• We convert these values into units of the data, x, giving us 2 Confidence Limits which define a Confidence Interval.

​Based on this, we might summarize the results of the test by saying "With a 95% Level of Confidence, we estimate the Mean height to be 175 centimeters, with a Margin of Error of plus or minus 5 cm."