Homework #2: Calculating a correlation yearGDP/capitaODA (millions) NOTE: these have different scale so I wouldn’t graph them together, but you could do two separate graphs. Do they move together?

Calculating a correlation in Excel Click on an empty cell and click on Insert function. Choose Correlation (either Correl or Pearson will work) Highlight array #1 (i.e. highlight the entire GNP/capita series) and then choose the second array (i.e. tech, ed., etc). Both arrays should span the same time period and have the same number of observations

Interpretation (+) positive relationship between the two variables (1 would be a perfect correlation). They move together. (-) Negative relationship (-1 would be perfectly negative correlation). They move in the opposite direction 0 If it is close to zero, there is no correlation.

Testing for statistical signficance of RCTs

Are the results reliable? A T-test simply measures whether there is a reliable difference between two means Control group Average collection Treatment group Average collection

Calculating a t-test (for unpaired samples, unequal variance) T statistic= difference between groups Variance within groups

Excel's TTEST Excel takes the T statistic and the degrees of freedom (based on the sample size (N-2)) to calculate a p-value: a test to see if you results are statistically significant When you do TTEST in Excel, it will return the p-value

P values: The smaller, the more significant! Statistical significance Good*: If it is less than 0.1 then the two samples are statistically different There’s a 1/10 chance that the difference is due to chance Better**: If it is less than 0.05 There’s a 1/20 chance that the difference is due to chance Best***: Less than 0.01 There’s a 1/100 chance that the difference is due to chance

Using Excel to calculate a t-test Click on empty cell Insert function TTEST Choose first array (control) Choose second array (treatment) Choose 2 tailed distribution Choose 3 sample unequal variance Enter Result shows p value.

Excel calculation Calculate the following: 1.Mean of control, Mean of treatment 2.P-value of TTEST Cans of food collected controltreatment total2942

Excel calculation Calculate the following: 1.Mean of control, Mean of treatment and Pvalue of TTEST > 0.10 so NOT statistically significant at 10% level Cans of food collected controltreatment totaltotal2942