# PSY 203 Lecture Notes - Lecture 2: Level Of Measurement, Central Tendency, Normal Distribution

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Lecture 2 Descriptive Stats

Definitions

• Population - the complete set of individuals, objects, or scores that the investigator is interested

in studying.

• Sample - a subset of the population.

• Parameter - a numerical characteristic of the population.

• Statistic - a numerical characteristic of a sample.

Measures of Central Tendency

• Mean

o Average of your data

o Most stable measure of central tendency

o Can be used to infer info about the population

o It is sensitive to extreme scores in the distribution

o The sum of the deviations about the mean equals zero

o The mean is least subject to sampling variation

o In replications, the mean would be the most similar

o sum of deviations would equal to 0

• Median

o The middle value where 50% of the score fall above and 50% of the scores fall below

o Even value of scores

▪

= middle value

▪

= median

• Mode

o Most frequent number in the set of data

o Used when you have nominal data

Relationship between these three measures of Central Tendency

Normal Curve

Negatively skewed Distribution

Positively Skewed Distribution