解析里面Z代表什么呢？

问题如下：

An analyst determines that approximately 99% of the observations of daily sales for a company are within the interval from $230,000 to $480,000 and that daily sales for the company are normally distributed. If approximately 99% of all the observations fall in the interval μ±3σ, then using the approximate z-value rather than the precise table, the standard deviation of daily sales for the company is closest to

选项：

A.$83,333 B.$41,667 C.$62,500

解释：

Given that sales are normally distributed, the mean is centered in the interval. MeanUnder a normal distribution, 99% of the observations will be approximately plus or minus three standard deviations. Next, use the following formula:

Z=(X-μ)/σ or, by re-arranging: σ=(X-μ)/Z,

where Z = 3,

X = $480,000, and

μ = $355,000.

Thus, ($480,000 – $355,000)/3.0 = $41,667.

Alternatively, use Z = –3, X = $230,000, and μ = $355,000: ($230,000 – $355,000)/(–3.0) = $41,667.