Veterinary Epidemiology and Zoonosis

Box and Whisker Plot, Shewart chart and Cusum chart

BOX AND WHISKER PLOT, SHEWHART CHART AND CUSUM CHART

Box and Whisker plot

• The frequency distributions (variations in a variable in different groups) can be compared by constructing Box and Whisker plots.
• The central horizontal line indicates the median.
• The upper and lower extremities of the vertical lines are known as Whiskers and these indicate maximum and minimum values of the data.
• The horizontal sides of the large rectangle that form the box are called Quartiles.
• Box and Whisker plots can detect any significant negative or undesirable deviations in the values from the standard / accepted values thus, enabling to take appropriate control measures.
• The numerical data can be displayed by this method.

Shewhart charts

• These charts can monitor any variation in a parameter over a period of time by plotting values graphically with time as the horizontal axis. In these charts, a standard / reference value is added as a horizontal line.
• Decision boundaries are the interference levels are also drawn on the chart.
• Remedial action are taken if the values of a variable crosses these boundaries.
• The decision boundary can be derived mathematically and is based on estimated standard error of Mean.
• These charts are effective in detecting large and abrupt changes in the values of a variable.

Cusum charts

• The cumulative sum technique is a sensitive method to detect small changes in a variable.
• There can be slight shift or drift away from the reference or standard value over a period of time. Such changes are detected when compared with control charts. This technique is known as Cusum charts.
•  The technique is useful in detecting changes in average levels, determining the point of onset of these changes, obtaining reliable estimates of current average values and making short term predictions of future average levels.