If the results from a number of tests are divided into groups having a certain “range” and the number of specimens having certain properties is plotted against that property, a frequency diagram is obtained. This is called the Normal Distribution Curve. Much of the variations found in nature and in industry follow the normal curve. The height of humans, the speed of antelopes, the results form an examination will be expected to follow the normal curve.

As the number of results is increased, the distribution of results will follow the normal curve more and more closely. The shape of the normal curve depends on the accuracy of the observations. Where conditions are not closely controlled a large number of readings will be further away from the average value, and the curve will be flattened. With rigid control most of the readings will be grouped within a small range and a sharp curve will be obtained.

The factor that determines the shape of the normal curve is known as the Standard Deviation. It should be pointed out that statistics is the science of large numbers, and can only be applied with fair accuracy where a reasonable number of observations are made.

The normal distribution curve can be used for assessing the accuracy of observations and for predicting the number of observations that will differ from the average value.

Experience on actual construction sites has shown that in general the conditions necessary for various degrees of control are as follows:

1. Good Control.

· Continuous supervision is done during the operation.

· Systematic testing of all materials is executed.

· Accurate measurements of measurements are used.

· Site laboratory is available.

· Quality control charts and records are maintained.


2. Average Control.

· Inspections are done at intervals of not more than 48 hours by knowledgeable technician.

· Batching, processing and application is supervised by suitably qualified artisans.

· Reasonably accurate methods of measurement are used.

· Frequent testing is done.

3. Poor Control.

· Inspection is done at weekly intervals.

· Rough measurements are used.

· Occasional tests are done.

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