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1. Make sure you arrange the data in order before picking out the median.
2. Make sure that you can calculate the mean correctly from a frequency distribution. Make sure you use the
frequencies when you calculate the mean.
3. Make sure that you know how to "undo" coding when you have calculated the mean of coded data. If data
has been coded using y = a + bx , then the mean of the coded data is related to the mean of the original data by the
equation y = a + bx .
4. Make sure that you know how to interpolate. When working with grouped data, you need to be able to use
linear interpolation to estimate the median.
1. Make sure that you know how to interpolate. When working with grouped data, you need to be able to use
linear interpolation to estimate quartiles or other percentiles.
2. Don't confuse variance with standard deviation. The standard deviation has the advantage that it is measured in
the same units as the data. The variance will be in square units. Make sure you read the question carefully and give the
3. Choose the appropriate form for the formula for Sxx. Remember that there are two versions of the formula for
Sxx. If the data set is fairly small and the mean is a round number, the form Sxx = (x - x)2 is quite easy to use.
However, if the mean is not a round number, then it is better to calculate x2 (in an examination question you might
be given this) and use the form Sxx = x2 = nx2 .
4. When working with frequencies, set your calculations out clearly. If the data is given in the form of a frequency
table, it is best to copy the table and add rows / columns to give the values of x², fx and fx², and the totals for f, fx and
fx². If the data is grouped, you need an extra row or column for the mid-point of the interval.
5. Avoid rounding errors. Use the memory function of your calculator to avoid rounding errors. For example, if the
mean of a set of data is not an exact number, store this in your calculator so that you can use it in the calculation of Sxx.
6. Be careful when using calculator statistical functions for finding the variance and standard deviation.
Different makes of calculator use different notations you need to be clear which is the appropriate key to use for
population standard deviation. It is often denoted by or n . (A different measure, sample standard deviation is
often denoted by s, n-1 or ^ ).
7. Make sure that you know how to "undo" coding when you have calculated the mean and standard deviation
of coded data. If data has been coded using y = a + bx , then the standard deviation of the coded data is related to
the standard deviation of the original data by the equation y = bx .
Example Given that y = x-2.5
10 and that y = 3.82 , find x .
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Wrong: x = 10y + 2.5 = 10×3.82 + 2.5 = 40.7
Right: x = 10y = 10×3.82 = 38.2
8. Do not confuse variance and standard deviation when using coding methods. If data is multiplied by k, the
standard deviation is multiplied by k, but the variance is multiplied by k2 .
Example Given that y = x-8.5
5 and that y = 1.24 , find x .
Wrong: 2x = 52y = 5×1.24 = 6.2
Right: 2x = 52y = 52×1.…read more
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Understand what is meant by mutually exclusive events. Mutually exclusive events cannot both occur at the
same time. For example, "getting a 3" and "getting a 6" when you throw a die are mutually exclusive, whereas
"getting an even number" and "getting a multiple of 3" are not. Remember that for mutually exclusive events A and B,
P (AB) = 0 , so the addition law becomes P (AB) = P (A) + P (B) .
11. Understand what is meant by independent events.…read more
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Define your variables. Carefully define your non-standardised variable with X or Y or..... (but of course not Z).
4. Be careful to distinguish between values of Z and values of X. Confusion with notation makes it harder for you to
be awarded method marks. Show clearly how you are standardising values.
5. Write down clear probability statements. Again you are more likely to receive method marks if your statements
are easy to read.
6. Give answers to probabilities to 3 significant figures.