Could you please help me understand HOW to find a percentile in laymans terms PLEASE.
In layman's terms, the percentile of a certain score is the percent
of lower scores than that certain score.
Mean temp 98.2 with standard deviation of 0.62, what is the body temp in the 95th percentile.
To get our thinking straight:
We start out by drawing the normal corve,
Now let's shade approximately the left 95% of the graph,
like below, where the green shaded part represents the
lower 95% of the graph. You don't have to be exact at all,
just guess about where the lower 95% would be.
There are two kinds of normal tables, the user-friendly
kind and the user unfriendly kind.
If you have the friendly kind:
You will find .9495 listed at z=1.64 and
.9505 listed at z=1.65. They are the
entries as close as you can find to .95
Since .95 is exactly half-way between
.9495 and .9595, we will use z=1.645 which
is exactly half way between z=1.64 and
z=1.65.
If you have the unfriendly kind:
You will subtact .95-.5 = .45
You will find .4495 listed at z=1.64 and
.4505 listed at z=1.65. They are the
entries as close as you can find to .45
Since .45 is exactly half-way between
.4495 and .4595, we will use z=1.645 which
is exactly half way between z=1.64 and
z=1.65.
Now we must convert a z-score to an x-score
by the formula:
x = 
+ 
z = 98.2 + (0.62)(1.645) = 99.2199.
If you have a TI-84 you can find it
directly
Press ON
Press CLEAR
Press 2nd
Press VARS
Press 3:InvNorm(
If yours is a newer 84 you will get this wizard.
Change the values to make it read this way
invnorm
area: 95
:98.2
:.62
Paste
Then highlight Paste and press ENTER
You will read
invNorm(.95,98.2
,.62)
If yours is an older 84 you will get
InvNorm(
and make it look like the above.
Then press ENTER
You will read 99.21980925
The calculator is a little more accurate than the table.
Edwin
