Need help on how to input invnorm and normalcdf using large numbers

Click For Summary

Discussion Overview

The discussion revolves around the use of the TI-83 calculator for statistical functions invNorm and normalcdf, specifically in the context of a problem involving normally distributed household incomes. Participants explore how to input large numbers and verify calculations related to income percentiles.

Discussion Character

  • Technical explanation
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about the correct syntax for using normalcdf and invNorm with large numbers in the context of a problem about household incomes.
  • Another participant suggests that the syntax for normalcdf and invNorm appears correct but does not provide definitive verification.
  • A participant claims to have calculated the percentage of households with incomes between $30,000 and $50,000 as 0.3824 and the minimum income for the top 90% as $66,800.21, expressing doubt about the latter result.
  • Another participant agrees with the first calculation but questions the second, suggesting that the answer may be correct based on their understanding.
  • One participant cites an answer key stating that the minimum income for the top 90% is $15,564, indicating a misunderstanding of the question regarding percentiles.
  • A later reply clarifies that the original question was asking for the 10th percentile, not the 90th, and provides the corrected syntax for invNorm.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct answers for the calculations, with conflicting results presented. There is also a misunderstanding regarding the percentile being asked for in the problem.

Contextual Notes

There is a lack of clarity regarding the interpretation of percentiles in the problem, leading to different answers being proposed. Additionally, the syntax for calculator functions is debated without definitive resolution.

aprilryan
Messages
20
Reaction score
0
Hi,

I'm a little unsure how to input large numbers into the TI-83 calculator using invNorm and normalcdf. Here's the question to the problem:

A study of VCR owners found that their annual household incomes are normally distributed with a mean \$41,182 and a standard deviation of \$19,990.

a. What is the percentage of households with incomes between \$30,000 and \$50,000?

Of course this cannot be inputted as normalcdf (30000, 50000, 41182, 19990) can it?

b. If an advertising campaign is to be targeted at those VCR owners whose incomes are in the top 90%, find the minimum income level for this target group.

This one also cannot be invNorm (.90, 41182,19990) can it?

Thanks
 
Physics news on Phys.org
Hi aprilryan,

a) I hadn't practiced this on the TI-83 before reading your question, but it seems this would be the correct syntax. What do you get for your answer? I can check it on another program.

b) Yes I believe this is the correct syntax. Again, I can check your answer if you wish.
 
The answer for a is .3824. The answer for b is 66800.21 (which I can sense is wrong). Yes, could you please check the answers for me? Thanks again.
 
aprilryan said:
The answer for a is .3824. The answer for b is 66800.21 (which I can sense is wrong). Yes, could you please check the answers for me? Thanks again.

I got the same answer for both! :) Looks good to me. Why do you think (b) is wrong? It's good to check the reasonability of an answer, so questioning your result is smart but in this case it is in fact correct.
 
Well, I checked the answer key and for part b it says the answer is $15,564. Thanks again.
 
aprilryan said:
Well, I checked the answer key and for part b it says the answer is $15,564. Thanks again.

Sorry I read the question incorrectly. It's asking you to find the top 90%, which is the same as the 10th percentile. When I read it I thought it was asking for the 90th percentile. So if you change your code to this it should work:

Code: 
 invNorm (.10, 41182,19990)
 
I got it now! Thanks!