# Equation

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For the enclosed, I need help figuring out the following:

What were the sales in 1988?

What will the sales be in 1993?

How much do the sales change per year from 1984 to 1992?

What's the 3 year moving average for 1984?

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##### Solution Summary

Product sales since 1984 are:

Year 1984 1985 1986 1987 1988 1989 1990 1991 1992

Sales 266 264 145 205 139 98 94 94 128

The least squares trend equation using the coded method is given:

Y1 = 159.22 - 21.18X, where X is set equal to 0 for 1984

###### Education

- BSc , Wuhan Univ. China
- MA, Shandong Univ.

###### Recent Feedback

- "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
- "excellent work"
- "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
- "Thank you"
- "Thank you very much for your valuable time and assistance!"

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