This is Stephanie from StatisticsHowTo.com, and in this video I will be showing you how
to calculate a normal distribution probability in Excel.
Got a question here concerning bacteria counts in a city's water supply.
In general with these types of normal distribution word problems, you will be given a mean, a
standard deviation. And the questions will ask you either the
less than probability, the more than or the between.
And I am going to show you how to do all three in Excel.
Less than 600, we can use the normdist function. So click on an empty cell, click on the insert
function button. Type normdist, N-O-R-M-D-I-S-T, and click
go. This gives you a list of functions, normdist
should be up the top, click OK. Our x-value, we wanted less than 600, so
600 is our x-value. The mean was given in the question as 500.
The standard deviation was given as 100. And type true in the cumulative box.
The only time you want to type false there is when you have a single value that you want
to find a single probability for, you are not likely to be doing that very often.
Click the OK button, and there’s the answer for the first question.
This is our less than probability .84. The second part of the question is asking
us, what is the probability of the bacteria count will be more than 600?
Let us going to show you a quick sketch I did earlier.
This blue area is the probability of up to 600, that’s what just what we calculated.
What we are interested in is this white area here.
We see the value under the curve for the total probability is always one.
All we have to do is subtract this blue area from one.
So I'm going to go back to the work sheet. Type the equal sign for a formula, one minus
the result that we just calculated. I am clicking on that cell, press enter and
this is the probability for more than 600. Finally, we want to figure out this between
probability and the question is asking us, what is the probability that the bacteria count
will be between 400 and 600? The easiest way to visualize this is to look
at a sketch. Here is the sketch of what we are looking
for. This is between the probability, between 400
and 600. We already calculated up to 600 and we already
calculated this white area over 600. Well the normal distribution graph is symmetrical.
So this white area here above 600 is going to be the same as this white area below 400
that is because 400 is one standard deviation away.
And 600 is also one standard deviation away from the mean.
So we don’t have to use normdist again if this is a different number you could do another
normdist. But all I am going to do is subtract the
larger value we calculated from the smaller value and that will give us the between.
So the answer to question number three is going to be this first value of less than
600 minus the second value of more than 600 which also equals less than 400.
And this is the probability for between 400 and 600.
That is how to calculate normal distribution probability in Excel.
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