I need to find the IQR of WAIS scores with mean 110 and sigma 25 (normal distribution). Will my calculations be Q1 = 25 , Q3 = 75 so..25-110/25=-3.4 (z) = z score of .0003 and 75-110/25 = -1.4 (z) and .0808 (z score). IQR = .0808 - .0003 = 0.0805?

Question

I need to find the IQR of WAIS scores with mean 110 and sigma 25 (normal distribution).   Will my calculations be Q1 = 25 , Q3 = 75 so..25-110/25=-3.4 (z) = z score of .0003 and 75-110/25 = -1.4 (z) and .0808 (z score).  IQR = .0808 - .0003 = 0.0805?

amistre64 · Answer

IQR deals with 25% to 75% of that data

it looks like you may have confused some of the parts tho

amistre64 · Answer

$$Z_{.2500}=\frac{x_1-mean}{\sigma}$$
$$Z_{.7500}=\frac{x_2-mean}{\sigma}$$

you have to find the z scores and then aglebra out the x1 and x2 parts

amistre64 · Answer

the zscores can either be found with an invNorm function, or from your author approved ztable

anonymous · Answer

I used Table A with my text to find that -3.4 = .0003 and -1.4 = .0808.  IQR = .0808-.0003 = 0.0805

amistre64 · Answer

thats not correct, you are trying to assume the x1 = .25  and that x2 = .75  and finding zscores ... that is completly backwards to the process you need to do

amistre64 · Answer

what zscore has a value closest to:  .2500 ?

what zscore has a value closest to:  .7500 ?

actually, when you find one the other is just the negation

amistre64 · Answer

and can you tell me the legend that your table is using?  is it a left tailed shade, or a "to the middle" shade?

anonymous · Answer

Ok...I found 2514 = -0.67 and 7517 = -0.68.  The table entry for z is left tailed shade.

anonymous · Answer

So, my computations should be:  Q1:  (-0.67) (25) + 110 = 93.25 
Q 3 = (0.68) x 25 + 110 = 126.75.  Q3 - Q1 = 126.75 - 93.25 = 43.5

amistre64 · Answer

youve done well,  but lets refine this.  we want "the closest value" to .2500

so we have to determine which option:  z = 0.67  or 0.68 is ore apt

.7486 < .7500 < .7517  subtract .74 from all the parts

.0086 < .0100 < .0117
 -14                      +17

since 14 is closer than 17 ... we should use the z=0.67  scores,  positive and negative

amistre64 · Answer

i meant to use the .7500 but my fingers hate me :)

youll see the -0.67  is closest to the .2500 by the same comparison

amistre64 · Answer

$$z=\frac{x-mean}{\sigma}$$solve for x
$$z(\sigma)+mean=x$$
$$-0.67(25)+100=x_1$$$$0.67(25)+100=x_2$$

then determine the range between x1 and x2

amistre64 · Answer

ugh ... and that 110, not 100

amistre64 · Answer

im getting 93.25  and 126.75,  giving me an IQR of 33.5

anonymous · Answer

I just finished and got that also.  I did this problem last week using the 0.67 values but found something in my text that confused me... again........Thank you ...again...I consider you a friend and hope you don't mind

amistre64 · Answer

youre welcome, and i dont mind :)  good luck