Chapter 8 Are Your Curves Normal? Probability and

Why it Counts

Part IIITaking Chances for Fun and Profit

Why Probability?

Basis for the normal curveProvides basis for understanding probability

of a possible outcomeBasis for determining the degree of

confidence that an outcome is “true”Example:

Are changes in student scores due to a particular intervention that took place or by chance alone?

The Normal Curve (a.k.a. the Bell-Shaped Curve)

Visual representation of a distribution of scores

Three characteristics…Mean, median, and mode are equal to one

anotherPerfectly symmetrical about the meanTails are asymptotic (get closer to horizontal

axis but never touch)

The Normal Curve

Hey, That’s Not Normal!In general, many events occur right in the

middle of a distribution with few on each end.

More Normal Curve 101

More Normal Curve 101

For all normal distributions…

almost 100% of scores will fit between -3 and +3 standard deviations from the mean.

So…distributions can be compared

Between different points on the X-axis, a certain percentage of cases will occur.

What’s Under the Curve?

The z Score

A standard score that is the result of dividing the amount that a raw score differs from the mean of the distribution by the standard deviation of the distribution.

What do these symbols mean?

The z Score

Raw scores below the mean have negative z-scores

Raw scores above the mean have positive z-scores

A z score is the number of standard deviations from the mean

z scores across different distributions (with different means and std devs) are comparable

What z Scores Represent

The areas of the curve that are covered by different z scores also represent the probability of a certain score occurring.

So try this one…In a distribution with a mean of 50 and a

standard deviation of 10, what is the probability that a score will be 70 or above?

What z Scores Really Represent

Knowing the probability that a z score will occur can help you determine how likely it is that a factor other than chance produced the observed outcome.

Keep in mind… z scores are typically reserved for populations

Hypothesis Testing & z Scores

Any event can have a probability associated with it.Probability values help determine how

“likely” or “unlikely” the event might beRange from 0 to 1The key --- if something has less than 5%

chance of occurring, we have a significant result

Using the ComputerCalculating z Scores