# TB3 Lecture 1; Key concepts from TB3/4 + SEM

• Created by: mint75
• Created on: 30-04-15 16:38

## Measures of central tendency

• Mean
• This an average of a set of scores.
• Used for interval or ratio data
• But, it can be affected by extremes/outliers
• Median
• This is the 'middle score' of an ordered set.
• Used for ordinal, interval and ratio data.
• Is resistant to extreme values and outliers.
• Mode
• The most frequent value in a set of scores
• Use any kind of data!
• Range
• The difference between the highest and lowest values in a data set.
• Variance
• The variability of scores
• Calculated by the sums of deviations from the mean(squared) divided by n-1.
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## STD DEV and SEM

• Standard deviation represents the difference from the mean, how much scores differ from the mean.
• Standard Error of the Mean (SEM) is how well the sample mean estimates to the population mean. Therefore variation in the sample = the 'error' in estimating the population mean.
• The equation for SEM is STDDEv of the sample/square root of N.
• A larger sample will yield a smaller SEM value.
• SEM is used to calculate confidence intervals and error bars in graphs.
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## The Normal Distribution

• This is a family of distribution, with the area under the curve representing 100% of the sample.
• The bell curve (aka the normal curve) can be seen with continuous variables, with scores closer to the mean appearing more frequent.
• The most extreme values are found at the tails of the curve, the tails approach x but never actually touch.
• The mean, mode and median are all very similar.
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## Sample types

• Smaller samples
• Lead to HIGH sampling errors
• Relatively varied sample means
• Larger samples
• Lead to LOW sampling errors
• Have generally stable sampling means
• The sample distribution of the mean is a normal distribution.
• Larger samples mean that there are smaller confidence intervals. This is because there need to be more specific estimates.
• A confidence interval is an interval estimate of the population parameters. 95% of the area under a normal curve is in the interval 1.96 SD away from the mean.
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