This table gives data from a
study of vampire bats feeding on cows.
During the study, cows were either in estrous (i.e. reproductively
receptive) or not, and were either bitten by a vampire bat or not.
To use the app, select a type of probability you
want to calculate from the drop-down menus - Marginal, Joint, or
Conditional.
Bat bite
Estrous
Bitten
Not bitten
Total
In estrous
15
7
22
Not in estrous
6
322
328
Total
21
329
350
Probability calculation
Marginal probabilities
Marginal probabilities are probabilities of an outcome on just one
variable. You selected , so the
marginal probability is the marginal total for
divided by the grand total of 350. This is the probability that a
randomly selected cow will be .
Joint probabilities
Joint probabilities are the probability of two events happening at
once. Your first selection is . Now
select an outcome on the other variable.
Conditional probabilities - selected the given first
Given means known to have occurred. By selecting
you are saying that you already know that the cow is ,
which is thus not subject to random chance. Since you already know this
outcome, only the data on cows that were
matter, so only those cells are highlighted. To calculate a conditional
probability you now need to select the conditional, which is the outcome
that is still subject to chance.
Conditional probabilities - selected the condition first
Selecting means that you want
to know the probability of
given that there is a known outcome on another variable. To calculate
the conditional probability of ,
you now need to select a given, which will identify what outcome is
already known to have occurred.
Conditional probabilities
Conditional probabilities are probabilities of an outcome, given some
information that is already known to be true. Since we know the given
has occurred, we only need to consider the data for the given outcome
(shaded in gray). The probability calculated is thus the probability
that a cow is given that you already know
it is .
