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Calculating Percentiles Using the Normal Curve

The second part of the TV sales problem requires calculation of the 90th percentile  of the demand histogram. In mathematical notation, we are looking for a such that P(X>a)=.10. See Figure  4.4.


  
Figure: The 90th percentile: $P( X \geq a)=.10$
\begin{figure} \begin{center} \epsfig{file=N36890th.ps, width=3.5in, angle=-90}\end{center}\end{figure}

\fbox{ \parbox{5.5in}{ \vspace*{1ex} {\bf Using the TI-83:} Calculating normal p... ...e*{4em} and end with a parenthesis.\\ \hspace*{2em}4. [ENTER] \vspace*{1ex} } }

Example: To find the 90th percentile  of TV sales, type the inputs (.90, 36, 8). The answer is 46.25, or approximately 46.


Exercise: Find the following percentiles for TV sales. Guess the percentile first, then use your calculator. Was your guess close?
a. 80th percentile
b. 20th percentile
c. 50th percentile




2003-09-08