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Variation is the fluctuation in a process’s output. Every measured output may not be identically same and we may notice some variation between multiple readings. Statically it is denoted by Standard Deviation (σ) which indicates the spread of each data point in the data set from the mean/average value. Example: Consider a machine producing 3.000 mm. diameter bolts. But each bolt may not measure 3.000 mm. diameter exactly. Some can be 2.999 mm., while some can be 3.001 mm. and there are endless possibilities. The spread of the various measurements around the mean (3.000 mm) is called the standard deviation. Lower the Standard Deviation or Lesser the variation of the diameters indicates that the data points are closer to the mean and the process is better. Aim of a good process design is to minimise this variation. There are two types of Variations: Common Cause Variation Special Cause Variation This is a Random Variation and is natural for the process. As name indicates it is “common” to the process. Other terms : Noise, non-controllable variation, within-group variation, or inherent variation This is a Non - Random Variation or Assignable cause and is not part of the normal process. As name indicates it is “special” to the process and the variation can be assigned to a reason. Though the value of each point cannot be predicted, the range of this variation is predictable. This range is called the process width or the Control limits. This is unanticipated and sporadic. It is completely unpredictable. Common cause variation is introduced by intrinsic variation in the process - by the variation present in People, Information systems, Machines/Equipment, Measurement, Materials and Environment. Special cause variation is introduced by the external parameters such as Operator not available, Computer crash, Power Outage, Machine malfunction. Generally, the process remains in control i.e. within the control limits and no corrective action may be required. If process deviates the control limits, then corrective actions are required. The process goes out of control. Reason of variation should be identified, analyzed and corrected if possible. If unable to correct alternate solutions should be implemented. E.g. The average normal body temperature is generally accepted as 98.6°F (37°C). Per some studies the "normal" body temperature can have a range, from 97°F (36.1°C) to 99°F (37.2°C). So, if the temperature is within this range and the person is otherwise feeling normal, then he may not need any medication E.g. If a person’s temperature goes beyond this range then there are high chances that he has a fever and might need to take medication. Why they should be differentiated and how misjudging one of these as the other can create problems? Common Cause Variations may not cause a process to go beyond control limits and so corrective actions may not be required. If corrections are required, then it would be intrinsic to the process like checking on the Manpower, Material, Method, Measurement, Machine, and Environment, shifting the process mean, adjusting the variance and so on. It exists even after "Special Cause" is removed. Special Cause Variations always cause the process to go out of control. The reason for the variation or “what has changed?” should be identified and analysed. If possible to rectify then it should be corrected else an alternate solution should be implemented. Mistake 1: Attribute a variation to Special Cause, when it is actually a Common Cause. Impact – Over-adjustment when not required. If deviation from target is considered due to special cause and the mean is adjusted for the deviation, then the adjustment will become a cause for further deviations and will worsen the situation. Mistake 2: Attribute a variation to Common Cause, when it is actually a Special Cause. Impact – Ignoring the variation and not doing anything. A special cause actually shifts the process mean, but this was ignored and no action taken to correct it. This further increases the variability. Example: Consider a pizza delivery joint located in locality A and catering to locality A & B and running an offer of “delivery in 30 mins or free”. Pizza production time: 10 mins Delivery Boy travel time to locality A: 10-20 mins. (Common Cause Variation) Delivery Boy travel time to locality B: 15-25 mins. (Common Cause Variation) Issue: So, some of the deliveries to locality B are free of cost. Analysis: This is Common Cause Variation. If the manager considers this as common cause variation, he can either continue with a few free deliveries (if it is not heavy on the business) or try to improve the pizza production cycle time But, if the manager commits Mistake 1 (i.e. considers it a Special Cause while it is a Common cause) then he may consider excluding locality B from the offer, which would have a greater impact on the whole business. Now consider the same scenario but with a new temporary condition: The road connecting locality A and B is undergoing renovation and there is frequent traffic delay of 10-15 mins. (Special Cause Variation) Analysis: This is Special Cause Variation. If the manager considers this as Special cause variation, he can decide to temporarily excluding locality B from the offer (or) modify the offer for locality B to “delivery in 45 mins or free” and communicate the valid reason. But, if the manager commits Mistake 2 (i.e. considers it a Common Cause while it is a Special cause), then he may: put in efforts to reduce pizza production time which would not resolve the issues (or) land up NOT taking any corrective correction. This could lead to significant increase in pizzas being delivered for free in locality B and unsatisfied customers in locality B