Insight 2010 Exam 1 core Q help

[lukaal340]

hey, I can't figure out what I'm doing wrong for q13 in core of Insight 2010 exam 1, any help would be appreciated.

A line of best fit for deseasonilised data was given as
Deseasonilised Montlhy Sales = 2500 - 1.2 x timecode where December 2009, t=1.
Predict the actual expected sales figures for January 2012 if the January seasonal index is 1.15.
A. 2470
B. 2468.8
C. 2839.12
D.2840.5
E.2146.78

to work it out, i went 2500 - 1.2 x 37 = 2455.6
2455.6 x 1.15 = 2823.94,
anyone see where im going wrong? thanks.

[lem]

Your time code should be 26 as its two years and 2 months

[lukaal340]

ahhh, ok. thanks.

[Yendall]

Just to step through it:











Sorry you probably knew how to do that, just felt like solving something :p

[lukaal340]

thanks for the answers, whilst this is here , question 10 of the core :
The distribution of times for the 500 meters running championship amongst high school students is bell shaped with a mean of 92 seconds and a standard deviation of 4.5 seconds.

Jordan is a high school student who attended the 500 meter running championship. If she ran 500 meters in 85.25 seconds, her standardised score (z- score) is closest to
A. 1
B. -1.5
C.-1.75
D. 1.75
E. 1.5

I used actual - mean / S.D to get -1.5, but flipped it to 1.5, as running 85.35 seconds is better than the mean, not worse, so answered E. the answer says B (-1.5). Do you not flip z - scores to make above 1 the better half, or is there a mistake in the answers? thanks.

[Yendall]

So you're given these statistics:






If one person runs 85.25 seconds, you must calculate how many standard deviations above or below the mean that would be equal to.



Now we know that running under 92 seconds is below the mean, making the z-score equal to a negative solution.

Now we have two choices, either -1.5 or -1.75. So we know that 1 standard deviation below the mean is 4.5, so 1.5 standard deviations below the mean would be 6.75.



Jordan ran 500 meters in 85.25 seconds, therefore he ran 1.5 standard deviations below the mean, resulting in a z-score of -1.5, thus the answer is B.

[lukaal340]

oohhh ok, i was thinking of it as shes 1.5 SDs better than the mean, therfore she has a +1.5 SD. but i see why its B now, thankyou again! haha

[Yendall]

It's easy to see stuff like that out of context. When dealing with raw data, the nature of the question doesn't really alter the statistics. I see why you would think about flipping it, but it doesn't work like that with z-scores as they are only a representation of standard deviations.

[paulsterio]

I used actual - mean / S.D to get -1.5, but flipped it to 1.5, as running 85.35 seconds is better than the mean, not worse, so answered E. the answer says B (-1.5). Do you not flip z - scores to make above 1 the better half, or is there a mistake in the answers? thanks.

You're thinking into it too much, just leave it as is

[Stick]

The fact that you're caught up in this question suggests to me that you don't fully understand the theory behind it all yet. Further is specifically designed so that the arithmetic is kept simple - it's more about comprehending the concepts (especially in core). Try not to over-think the questions too much.

[lukaal340]

ok thanks guys