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Accident Records and Injury Rates less than four because the lower limit will be negative Since a plant cannot experi ence a negative number of injuries per month, a chart based on an average of less than four would not yield meaningful results 3 Calculate a new average each year to reflect any change in the monthly average number of injuries 4 Construct more than one chart if there are seasonal changes in employment For example, if an operation employs one hundred people for the first 6 months of each year and four hundred people for the last 6 months, a separate chart must be constructed for each 6-month period 5 Construct a new control chart if external factors, such as a change in the level of employment, a change in the work environ ment, or a change m work hazards, bnng about a permanent change in work-injury experience When calculating a new aver age m such a situation, do not use experi ence prior to the change References American Standards Institute 1430 Broadway, New York, N Y 10018 Method of Recording and Measuring Work Injury Experience, Z16 1 -- 1967 Method of Recording Basic Facts Relating to the Nature and Occurrence of Work Injuries, Z16 2-1962 Walker, Helen M , and Lev, Joseph Elementary Statistical Methods, rev ed New York, N Y , Holt, Rinehart and Winston, 1958 266