Help!

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Help!
Message posted by Angie on February 11, 2001 at 12:00 AM (ET)

There is a set of 50 scores, the mean equals 12.38 and the SD equals 4.26. Construct a SD spread figure based on these data. Be sure to specify how many data vaules are in each intercal of the figure.

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Re: Help!
Message posted by CY on February 12, 2001 at 12:00 AM (ET)

Well, since the meand and the SD are given, because 66% of the data would be within 1 SD of the mean while 95% would be within 2 SD of the mean, you can then try and work out the percentages for the total number of data avaliable, plot a graph which should come out as a bell shape to show normal distribution, okay? I hope you can crack this.

Re: Help!
Message posted by nancy diehl on February 12, 2001 at 12:00 AM (ET)

I will assume that your data are normally distributed, in which
case you can use the empirical rule for a normal distribution. The
empirical rule states that given a normal distribution, you should
find the mean +/- 1 std dev. to be represented by 68.26% of your population.
The mean +/- 2 std dev. should represented by 95.44% of your population
and lastly, the mean +/- 3 std. dev. should represent 99.74% of your population.
So, you need to calculate these lower and upper bounds and see how many
fall within each interval (assuming you have the actual values of the 50 observations.
If not, then you have to multiply the percentages above by 50. By the way,
the percentages of 68, 95 & 99, come from your Z-table. Look under
a Z value of 1, 2, 3. You will find the area under the curve equates to .3413, .4772 and
.4987, respectively. You double the area since you are looking at an interval
on both sides of the mean.

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