retired_Angela Adams

Well it's definitely a statistics module, I was just interested to know what course it was part of.

retired_susan mcdonald

it's just a bit of management science i have to do along with marketing.

retired_khabmth

</BLOCKQUOTE>

Thought you needed the actual values to find quartiles but it was a while ago.

15 observations (right?) would give Q1 at 15/4 and Q3 at 3*(15/4) and the interquartile range should be the difference between the two values. But as I said it was a long time since I did box-and-whisker plots.

retired_nini59

i've got the 1st quartile as 6. Now my notes are a bit wrong I think, dunno. but to find the 3rd quartile would i do 5+((6*5)/5)=11

is that correct? I've used the L+K*W/F formula </BLOCKQUOTE>

if there's 15 in total, then it's 15/4 = 3.75, and 15/4*3 = 11.25, so find out where those values would fall.

The first one is 0.75 into the 5-10 range, as there's 5 in there it's (10-5)/5*0.75+5 = 5.75 I think, the second one is 3.25 into the 10-15 range, so it's (15-10)/6*3.25+10 = 12.7

12.7-5.75 = 6.96

retired_khabmth

Looks about right. :thup:

retired_Angela Adams

The formula for the upper quartile is

(3n/4)+(1/2)

Since n=15, the equation gives 11.75 so the upper quartile is the 12th value. Goin off symmetry, the lower quartile is the 3rd value.

Now using linear interpolation you can estimate the respective values.

The general formula is

(group width/ frequency)*(the place of the value you're looking for)

The 3rd value = (5/3)*3
The 12th value = (5/6)*4

Might be better to sketch a graph if you don't get the last bit.

retired_Angela Adams

or not /o\

retired_Angela Adams

Lower quartile should ahve been a 4 on my method too

retired_khabmth

Where did you get the "+(1/2)" from???

retired_Angela Adams

It's just a standard way of working out which value you should be looking for. Say

(3n/4)+(1/2)=X

If X is an integer than that is the quartile value. If the fractional part of X is (1/4), round it down, if 3/4, round up. And if the fractional part is 1/2 the quartile value is the average of the (X+1/2) and (X-1/2) values.