Special Control Charts - Hotelling multivariate chart |

Hotelling control chart is used when several correlated variables are controlled simultaneously. Within this module, the data are assumed to consist of individual (multivariate) data points, hence the chart is a multivariate analogue of the X-individual chart. Such data might be used in X-individual control charts for separate variables only if the variables are not correlated – their correlation coefficients should not be significant. Hotelling chart reduces the information about individual variables to one characteristic, namely distance from the (multivariate) mean computed with respect to the covariance matrix (Mahalanobis distance). The distance cannot be negative and respects interrelationships among different variables. The lower limit is always set to zero.
Extended control charts - Pdf manual
Example
Task: Monitoring the process of pressing tablets in pharmacy plant
Data:
Output:
Hotelling chart for multidimensional data can find problems and unstabilities tha classical Shewhart chart cannot see, as Hotelling charts take into account also correlation structure of the process. |