standard deviation

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standard deviation
Message posted by jon allen (via 208.61.242.6) on September 12, 2001 at 4:42 PM (ET)

If the mean is 10 with a standard deviation of 3,would one SD above the mean be: 13-15? or 14-16? In other words, when determining standard deviation, do you include 10(mean)in the 3 point SD? or do you count three above (11-13) and three below (9-7)? As well, if the mean is 100, would one SD above the mean be at 114 or 115? One SD below the mean 84 0r 85? Thank you

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Re: standard deviation
Message posted by Darius (via 200.23.217.10) on September 13, 2001 at 10:04 AM (ET)

Sorry but, I tink you have no idea about what standard deviation is.

The standard deviation is a measure of the variation AROUND the mean value.

So if sd=3,mean=10, 10+1sd = 13

If you try to describe the 99.97% (taking in account that the data has a normal distribution behaviur) of the data you use mean +/- 3sd.

So mean+/-3sd >> 10+3*3=16, 10-3*3=4.

Re: standard deviation
Message posted by nancy diehl (via 129.176.151.122) on September 13, 2001 at 10:12 AM (ET)

If your mean is 10 and your stddev is 3, then one stddev above the mean returns a value of 13 (mean + 1{stdev unit}). One stddev below the mean would be 10-3, or 7. Hence, if your data follows a normal distribution, the empirical rule states that 68% of your population should fall between the mean +/- 1(stdev unit). That is for this example, 68% should fall between 7 to 13.

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