Statistics for Decision Makers - 04.03 - Restriction of Range

title

04.03 - Restriction of Range

author

Bernard Szlachta (NobleProg Ltd) bs@nobleprog.co.uk

IQ Example。

• An employer wants to check whether productivity in programming can be predicted from IQ scores
• He performs the test on graduates (people without experience) and tests it after a couple of years against their performance assessments
• The test shows no significant correlation between the IQ scores and programmers' performance

Can we conclude that there is no correlation between IQ scores and performance of programmers?

Note that selecting employees based on IQ test for certain types of jobs may be illegal.

IQ Example Explanations。

• Graduates usually have high IQ scores, i.e. they are already a subset from a bigger population
• Therefore if we do the test on a sample which is representative of the whole population, the result will be different
• If we preselect our sample (i.e. remove people with lower IQ scores in this case), the correlation usually is much lower or is even insignificant

More examples 。

Is weight correlated with footballers' performance?

Omitting Extremes。

Selecting Extremes。

Upper Crust。

Quiz 。

Quiz Time

Quiz

1 In which of the situations below can range restriction take place?

	Asking existing customers about product satisfaction.
	Asking randomly selected sample of people about the usefulness of a product.
	Determining correlation between SAT scores and job performance in a general population by testing Google employees.

Answer >>

Existing customers are just a subset of people who potentially want to buy our product. Google employees has been already preselected based on their SAT scores directly or indirectly.