Sampling & Flowcharting

November 29, 2009
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Statistical sampling and flowcharting are two more important tools in the arsenal of tools & techniques to be used for planning for quality. Sampling involves carrying out inspections on a small subset of a population of items/components or measurements carried out to find the state of health of the manufacturing or other processes as applicable. The samples are need to truly representative of the population from which they are drawn. There is a solid statistical foundation to the theory that governs what size of samples and at what frequency is to be taken to meet the above criteria. The personnel involved in quality planning will have to be aware of the appropriate techniques.

Flowcharting describes a process accurately depicting interactions of subsystems and the inputs. Thus this is a good tool to understand the product development or the process involved. To be of value the flowchart has to depict every block and their interaction that is significant as also the required inputs.

Statistical Sampling

Statistical sampling lets us gather knowledge about a population by selecting some individual responses. Such observation usually measures multiple properties of an observable entity. The properties are such that they help distinguish individuals. Weight, location, income, brand of the car he owns and such observable items can be the properties measured when a sampling is carried out on a section of human population, for example. The way samples are to be taken has foundation in statistics and probability sciences. The concern is always, as described earlier, in making sure whatever inferences are drawn from the sample population holds equally well with the larger population. All the statistical and probability theory foundations must establish that the way the sample is taken is truly representative. Any bias introduced into the conclusions drawn must be known and measureable, ideally non-existent.

Taking recourse to sampling also is a practical need. Many of the inspections cannot be carried out as the population is too large in many cases and it would be impractical as an inordinate amount of time may have to be spent on completing the inspection. In case of destructive inspections testing the complete population, even when it is may be possible to do so, is not advisable.

To be able to draw representative samples, the population must be accurately known. That may not look like a big problem on first thought. A batch of materials or physical components being the population is quite obvious. But when you are trying to determine some aspect of supermarket personnel, for example, the length of the queue waiting to be serviced may the population in which one is interested. The sampling procedure to be used, thus, must be defined at the planning time. The inspection time, cost of inspection and cost of scrap needs to be taken into account towards cost of quality.

Flowcharting

Flowcharting essentially shows the relationships between process steps. Typical process flowcharts show activities, decision points, the order of processing etc. What the flowcharts essentially can do is that it makes the understanding of the process under study very clear. The quality personnel thus would be able to anticipate quality issues, if any, that can happen. That will let them prepare for these eventualities.

The figure 8-7 in the PMBOK is a typical example of such a process flowchart. The rectangular boxes indicate activities related to the process. These are numbered and the text inside indicate the activity taking place. The direction of the arrows from one box to the next indicates the flow of the process. The decision points can make the flow change depending on the decision take at that point.

Additional Tools

There are many other quality management methodologies in use today and a particular organization may use one such. Six sigma, quality function deployment, CMMI are some such examples. Additional quality planning tools, matrix diagram and prioritization matrices may be used by specific organizations.

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