momto4kiddos
Drama Llama
Posts: 5,151
Jun 26, 2014 11:45:15 GMT
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Post by momto4kiddos on Jun 23, 2016 16:20:50 GMT
I can't figure where i'm going wrong on this. Wondering if someone can figure out what i've missed.
So it's steel rods. Mean is 29 cm, standard deviation 0.06 cm.
What proportion made is less than 28.9? I'm good with this, my answer comes out right
28.9 - 29, divided by 0.06 = 1.66 Look it up on the standard normal distribution table and the answer is 0.0478 All is good.
Here's the problem...question is that rods shorter than 28.87 or longer than 29.13 are discarded. What proportion are discarded? If I calculate each one the same as before 28.87 - 29 then divide by standard deviation, as well as 29.13 - 29, etc. I get 2.16, -2.16 answers. Which does not correspond on the chart to the answer I know is correct because it was given. The given answer is 0.0303
Can anyone see what i'm doing wrong, I know i'm missing something.
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Post by Merge on Jun 23, 2016 16:28:59 GMT
I'm sorry, none of this makes any sense to me!
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Post by Basket1lady on Jun 23, 2016 17:06:12 GMT
OMG, this is worse than the 7th circle of hell. Thank you God for the wisdom to have never taken statistics! Best of luck to you, OP.
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Post by elaine on Jun 23, 2016 17:24:37 GMT
I can't figure where i'm going wrong on this. Wondering if someone can figure out what i've missed. So it's steel rods. Mean is 29 cm, standard deviation 0.06 cm. What proportion made is less than 28.9? I'm good with this, my answer comes out right 28.9 - 29, divided by 0.06 = 1.66 Look it up on the standard normal distribution table and the answer is 0.0478 All is good. Here's the problem...question is that rods shorter than 28.87 or longer than 29.13 are discarded. What proportion are discarded? If I calculate each one the same as before 28.87 - 29 then divide by standard deviation, as well as 29.13 - 29, etc. I get 2.16, -2.16 answers. Which does not correspond on the chart to the answer I know is correct because it was given. The given answer is 0.0303 Can anyone see what i'm doing wrong, I know i'm missing something. I don't have Normal Distribution table handy. You are discarding all the rods that are greater than 2.16 SD's above the mean + all the rods shorter than -2.16 SDs according to a chart on the internets, the percent to either side of a Z of 2.16 and -2.16 is .0166 So, .0166 + .0166 = .0332 The very small difference between my answer and the teacher's may be due to differences in the charts used due to rounding. |
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Post by elaine on Jun 23, 2016 17:30:07 GMT
I can't figure where i'm going wrong on this. Wondering if someone can figure out what i've missed. So it's steel rods. Mean is 29 cm, standard deviation 0.06 cm. What proportion made is less than 28.9? I'm good with this, my answer comes out right 28.9 - 29, divided by 0.06 = 1.66 Look it up on the standard normal distribution table and the answer is 0.0478 All is good. Here's the problem...question is that rods shorter than 28.87 or longer than 29.13 are discarded. What proportion are discarded? If I calculate each one the same as before 28.87 - 29 then divide by standard deviation, as well as 29.13 - 29, etc. I get 2.16, -2.16 answers. Which does not correspond on the chart to the answer I know is correct because it was given. The given answer is 0.0303 Can anyone see what i'm doing wrong, I know i'm missing something. The Z scores should be -2.17 and 2.17. You rounded down 2.16666666... When it should have been rounded up to 2.17. .015 + .015 = .03 same as answer given to you. |
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cathit
Shy Member
Posts: 47 
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Post by cathit on Jun 23, 2016 17:35:53 GMT
Ok, I have no idea what any of this means, but I asked my husband who majored in statistics - this is what he says
2.16 is the z-score. If you then look up 2.16 in the normal distribution table, you get .01539. (Just like you got .0478 when you looked up the z-score of 2.16.) If you then take .01539 times two, you get .0308 as the answer. (The reason you multiply by two is that you want the percentage both below 28.87 and above 29.13.
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Post by Darcy Collins on Jun 23, 2016 17:43:59 GMT
Did you add the two together? I don't have a normal distribution table, but make sure you're adding them together to reflect that both the high and low are being discarded.
ETA - I guess that's what I get for opening a bunch of tabs and then getting distracted. read the posts above mine. They already covered it.
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momto4kiddos
Drama Llama
Posts: 5,151
Jun 26, 2014 11:45:15 GMT
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Post by momto4kiddos on Jun 23, 2016 18:06:21 GMT
Thanks everyone... I've done a few different problems and have been close so I think it's in my rounding. I'll rework it in a bit, i'm taking a mental health break right about now  Counting the days until this class is behind me  |
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Post by elaine on Jun 23, 2016 18:10:46 GMT
Thanks everyone... I've done a few different problems and have been close so I think it's in my rounding. I'll rework it in a bit, i'm taking a mental health break right about now Counting the days until this class is behind me 4 and below are always rounded down and 6 and above are rounded up. Always ask your math instructor at the beginning of the course how he/she wants you to round 5. In stats it is usually rounded down rather than up so that you lessen the chance of finding statistical significance due to even minute inflations of your numbers, but different mathematicians have different preferences. |
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Deleted
Posts: 0
May 28, 2024 23:51:27 GMT
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Post by Deleted on Jun 23, 2016 18:41:38 GMT
Can I admit I actually miss statistics? I loved those classes!!! (part of an Applied Mathematics degree). I don't do much by way of statistics anymore though...
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Counting the days until this class is behind me 