More statistics!

Check in Statement of proposal due today Method section draft assigned –Participants Who will you survey or experiment on? What type of sample? –Procedures If a survey, how conducted (individually, in groups?) –Measures How will you measure things?

More check in… Quiz next wednesday It will cover experimental design –Internal validity and threats –External validity and threats –Experiments and types of pilot studies –It will cover statistics through today Same format as last quiz

Today Frequency distributions

What is a frequency distribution Frequency –The rate at which a score on a variable occurs in a sample

Frequency ScoreFrequency 1, 2 2, 2, 2,3 3, 3, 3, 34 4, 4, 4, 4, 45 5, 5, 5, 5, 5, 56 6, 6, 6, 6, 6, 66 7, 7, 7, 74 8, 8, 8,3 9, 92

Frequency distribution? How that frequency of scores is distributed or arranged

Frequency distribution This is sometimes called a frequency polygon

Types of frequency distributions The normal curve –Also called the Gaussian curve –The bell curve Describes how the data occur Their frequency

The normal curve

The normal curve? The rate of scores is symetrically distributed around the mean They follow a smooth increase/decrease with a peak at the mean, and frequencies of approximately 0 on either side

The normal distribution A lot of things are “normally” distributed Weight Height The normal distribution is important statistically –For reasons you don’t need to know right now

Other distributions? There are other important named statistical distributions—e.g. Poisson You don’t have to know them right now They are just different terms for talking about the way data are arranged Data—a plural term

A lot of data is not normally distributed It is skewed When data are “skewed” there are a lot more scores on one side than the other Income is a fine example It is positively skewed There are more scores on the “left” side of the distribution

Positive skew

Positive Skew Measures of mental illness symptoms like depression are usually positively skewed Most people don’t have many symptoms

Negative skew When there is more data on the right hand side of the distribution

Why is the normal distribution special? We can make probability statements about the occurrence of certain scores –If the data are normally distributed