standard deviation is a measure of precision or how well
individual measurements agree with each other. The formula
is:
where x1, x2 and x3 are the induvidual
measurements; x(bar) is the mean and n is the nuimber of
measurements.
Cumulative frequency curve like the standard
deviation, is a good way to measure dispersion.
It is
obtained by adding up the frequencies as you go along, to give a 'running
total'.
Plot the cumulative frequency as the
y-axis and the interval or measurements in the x
axis.
When looking at a cumulative frequency curve, you
will need to know how to find its median, lower and upper quartiles, and the
interquartile range.
By drawing horizontal lines to
represent 1/4 of the total frequency, 1/2 of the total frequency and 3/4 of the total
frequency, we can read estimates of the lower quartile, median and upper quartile from
the horizontal axis.
The interquartile
range is the difference between the lower and upper
quartile.
The box and whisker plot on the other hand is
used to display information about the range, the median and the quartiles. It is usually
drawn alongside a number line. Below is an example
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