Chapter 6.1 & 6.2
By: Cameron Duhon
Normal Curves and Sampling Distribution
Important Properties of a Normal Curve:
- The curve is bell-shaped, with the highest point over the mean u.
- The curve is symmetrical about a vertical line through u.
- The curve approaches the horizontal axis but never touches or crosses it.
- The inflection (transition) points between cupping upward and downward occur above u + o and u - o.
- The area under the entire curve is 1.
M & O
The parameters that control the shape of a normal curve are the mean u and the standard deviation o. When both u and o are specified, a specific curve is determined. In brief, u locates the balance point and o determines the extent of the spread
For a distribution that is symmetrical and bell-shaped (in particular, for a normal distribution):
- Approximately 68% of the data values will lie within 1 standard deviation on each side of the mean.
- Approximately 95% of the data values will lie within 2 standard deviations on each side of the mean.
- Approximately 99.7% (or almost all) of the data values will lie within 3 standard deviations on each side of the mean
How to make a control chart for the random variable x
A control chart for a random variable x is a plot of observed x values in time sequence order
- Find the mean u and standard deviation o of the x distribution by using past data from a period during which the process was "in control" or using specified "target" values for u and o
- Create a graph in which the vertical axis represents x values and the horizontal axis represents time
- Draw a horizontal line at height u and horizontal, dashed control-limit lines at u +- 2o and u +- 3o
- Plot the variable x on the graph in time sequence order. Use line segments to connect the points in time sequence order
- Out-of-Control Signal I: One point falls beyond the 3o level
- Out-of-Control Signal II: A run of nine consecutive points on one side of the center line (the line at target value u)
- Out-of-Control Signal III: At least two of three consecutive points lie beyond the 2o level on the same side of the center line
A standard score or z score of a measurement tells us the number of standard deviations the measurements is from the mean.
- A standard score close to zero tells us the measurement is near the mean of the distribution
- A positive standard score tells us the measurement is above the mean.
- A negative standard score tells us the measurement is below the mean.
Convention for Using Table 5 of Appendix II
- Treat any area to the left of a z value smaller than -3.49 as 0.000.
- Treat any area to the left of a z value greater than 3.49 as 1.000.