PSYCH 240 1st Edition Lecture 7Section 5: Probability cont.Probability-Being scientific means expressing an appropriate level of confidence in our beliefs-We try to get more evidence that justifies a high degree of certainty-If strong evidence is not available, the scientific thing to do is acknowledge our uncertainty Combining Probabilities-Addition Rule: the probability of getting any one of multiple mutually-exclusive outcomeis the sum of the individual probabilities of each outcomeoEx.-Multiplication Rule: the probability of simultaneously observing multiple independent outcome is the product of the probabilities of each individual outcomeoAlways express probabilities as proportionsoEx.Percentage Change-Percentage Change: the difference btw the new and old value of a variable divided by the old value (times 100 to make it a percentage)-PC = 100 x (NV-OV)/OVoPC = percentage changeoOV = old valueoNV = new value-If the value comes out positive, then it is a percentage increase-If it comes out negative, then it is a percentage decrease-Unlike regular percentages, percent change can be over 100%o100% increasing means doubling, 200% increase means tripling, 300% means quadrupling, etc.-When you have the old value and the percentage change, you can figure out what the new value iso% increase: NV = OV + (PC/100 x OV)o% decrease: NV = OV - (PC/100 x OV)-When you only have information on the percentage change and not the absolute change, you should be suspicious-If you don’t know the original value or the new value, then you can't figure out how big the absolute change was in the variable-The same percentage change can represent different absolute changes depending on the original value-This is especially tricky when the original variable itself is expressed in percentages-__% increase/decrease means something very different than an increase/decrease of ___ percentage pointsoEx. If 4% of people quit smoking without a therapy program and 6% quit with the therapy program, you can either say that there was a 50% increase in quitting or that the rate of quitting increased by 2 percentage points (advertisers will choose 50%)Probabilities-Marginal Probability: overall probability of an event in a populationoMarginal probability of A is denoted p(A)-Conditional Probability: probability of an event just for a subset of the population that has a certain characteristicoThe conditional probability of A given B is denoted p(B/A)oCan be different depending on which variable you use as a condition.P(A/B) doesnot equal p(B/A)-Contingency tables are a good way to demonstrate the difference btw marginal and conditional probabilitiesoShow the number of scores at each combination of the levels of multiple variablesoYou get the marginal probabilities by dividing the column or row totals by the grand totaloYou get conditional probabilities by dividing a cell total by a column or row total. The condition tells you which column or row total to use. The requested probability tells you which cell to