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

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  • 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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  • 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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