work out the mean, then for each number subtract the mean and square the result, then work out the average of the squared values.
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why do we square units?
if there are negative variances/ differences from the mean within the data the negatives could cancel out the positives, so squaring the values makes them all positive
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what does the variance mean?
shows us how spread out the data is
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how do you calculate standard deviation?
it is the square root of variance
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what does standard deviation mean?
gives us the standardised average amount of deviation from the mean.
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what percentage of values are within 1 SD of the mean?
68%
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what percentage of values are within 2 SDs of the mean?
95%
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what percentage of values are within 3 Sds of the mean?
99.7%
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what are z scores?
he number of standard deviations from the mean is also called the "Standard Score” or "z-score"
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Howard z scores calculated
we can calculate the z score of a given value by subtracting the mean from that value and then dividing this by the SD
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what is the mean of all z scores?
0
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what is the standard deviation of all z scores?
1
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what does a negative z score show? positive? zero?
negative = below mean, positive = above mean, zero = exactly average
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how can you tell if a z score falls at the extreme 5% of the distribution?
if it is above +/- 1.645
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what do z scores tell you?
how unlikely or extreme your result is in comparison to the rest of the data
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what are descriptive statistics?
mean, mode, median and SD.
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m
n
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Other cards in this set
Card 2
Front
why do we square units?
Back
if there are negative variances/ differences from the mean within the data the negatives could cancel out the positives, so squaring the values makes them all positive
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