A drug company has developed a new drug that is designed to reduce high blood pressure. To test a drug, a sample of 15 patients is recruited to take the drug. Their systolic blood pressures are reduced by an average of 28.3 millimetres of mercury (mmHg), with a standard deviation of 12.0 mmHg. Another sample of 20 patients takes a standard drug. Their systolic blood pressures are reduced by an average of 17.1 millimetres of mercury (mmHg), with a standard deviation of 9.0mmHg. Assume that blood pressure reductions are approximately normally distributed and true standard deviations for blood pressure reductions for both the drugs are equal. Write down the formula to calculate the 95% confidence interval for the difference between the true average reduction for the new drug and that of standard drug. Also, write down the values of all constants, parameters, statistics and critical values used in the formula.