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Biostatistics for Animal Science

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22 <strong>Biostatistics</strong> <strong>for</strong> <strong>Animal</strong> <strong>Science</strong><br />

Find the following probabilities:<br />

a) the first calf is spotted,<br />

b) the first calf is either black or red,<br />

c) the second calf is black if the first was spotted,<br />

d) the first calf is spotted and the second black,<br />

e) the first two calves are spotted and black, regardless of order.<br />

Solutions:<br />

a) There is a total of 10 calves, and 5 are spotted. The number of favorable outcomes are<br />

m = 5 and the total number of outcomes is n = 10. Thus, the probability that a calf is spotted<br />

is:<br />

P(spotted) = 5 /10 = 1 /2<br />

b) The probability that the first calf is either black or red is an example of union.<br />

P(black or red) = P(black) + P(red) = 2 /10 + 3 /10 = 5 /10 = ½. Also, this is equal to the<br />

probability that the first calf is not spotted, the complement of the event described in a):<br />

P(black ∪ red ) = 1 - P(spotted) = 1 - 1 /2 = 1 /2<br />

c) This is an example of conditional probability.<br />

The probability that the second calf is black if we know that the first one was spotted is the<br />

number of black calves (2) divided by the number of calves remaining after removing a<br />

spotted one from the pen (9):<br />

P(black | spotted) = 2 /9<br />

d) This is an example of the probability of an intersection of events. The probability that the<br />

first calf is spotted is P(spotted) = 0.5. The probability that the second calf is black when the<br />

first was spotted is:<br />

P(black | spotted) = 2 /9<br />

The probability that the first calf is spotted and the second is black is the intersection:<br />

P[spotted ∩ (black | spotted)] = ( 5 /10) ( 2 /9) = 1 /9<br />

e) We have already seen that the probability that the first calf is spotted and the second is<br />

black is:<br />

P[spotted ∩ (black | spotted)] = 1 /9.<br />

Similarly, the probability that the first is black and the second is spotted is:<br />

P[black ∩ (spotted | black)] = ( 2 /10) ( 5 /9) = 1 /9<br />

Since we are looking <strong>for</strong> a pair (black, spotted) regardless of the order, then we have either<br />

(spotted, black) or (black, spotted) event. This is an example of union, so the probability is:<br />

P{[spotted ∩ (black | spotted)] ∪ [black ∩ (spotted | black)]} = ( 1 /9) + ( 1 /9) = 2 /9<br />

We can illustrate the previous examples using a tree diagram:

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