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Table 1 Factors that affect sample size calculations

From: Statistics review 4: Sample size calculations

Factor Magnitude Impact on identification of effect Required sample size
P value Small Stringent criterion; difficult to achieve 'significance' Large
  Large Relaxed criterion; 'significance' easier to attain Small
Power Low Identification unlikely Small
  High Identification more probable Large
Effect Small Difficult to identify Large
  Large Easy to identify Small