# S1 crucial points

S1 crucial points for edexcel

- Created by: Maja
- Created on: 31-05-12 12:48

First 585 words of the document:

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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.

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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

answer requested.

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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