One-tail and two-tail tests

Description

When a set of measures is made, they typically appear as a distribution, with more 'average' measures in the middle and less at the extremes, as in the standard normal 'bell' curve.

Experimental assessment typically grabs the majority, snipping off extreme 'tails' as less likely and typically forming the acceptable 5% error. This is a two-tailed test.

Some tests seek to discover questions about 'more' or 'less' and, rather than snipping off the outer tails, draws a line and selects the people below or above the line. This is a one-tailed test.

Example

A two-tailed experiment seeks to understand the intelligence as measured by 'IQ' of a group of people and finds that 95% of the people have an IQ between 113 and 145. The other 5% are above or below these figures. In a Normal distribution, this would be around 2.5% below 113 and 2.5% above 145.

A one-tailed experiment starts with a 'genius' IQ rating of 150 and seeks to understand whether training can increase the number of geniuses in a group. The measure thus slices off the top section of the group, both before and after the treatment.

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One-tail and two-tail tests

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