Interpreting Interpreting the Modelthe Model

e-Counter e-Counter PlaguePlague•How many people are in this class?•How many people were initially infected?•How many are sick at this stage?•How many people have recovered?•How many were immune initially?

e-Counter e-Counter PlaguePlague•29 + 1 = 30 people in the class•1 was initially infected.•1 is sick at this stage.•1 has recovered.•No one was initially immune.

e-Counter e-Counter PlaguePlague•How many people are infected if the die gives a 1?

0•How about a 2?

0•3?

0•4?

1•5?

1•6?

1

e-Counter e-Counter PlaguePlague•If the die is thrown once, what infection rate should we expect?

3

6=

12

e-Counter e-Counter PlaguePlagueThe rate of infection has changed here, but everything else is the same.•If the die is thrown once, what infection rate should we expect?4

6=

23

e-Counter e-Counter PlaguePlague•If the die is thrown once, what infection rate should we expect?

•Can you explain this?•Does this mean someone is certain to be infected?

6

6=1

e-Counter e-Counter PlaguePlague•If the die is thrown once, what infection rate should we expect?

•Can you explain this?•Does this mean someone is certain to be infected?

8

6=1

13

e-Counter e-Counter PlaguePlague•Do you think this epidemic will take off, or die out soon?

e-Counter e-Counter PlaguePlague•We should expect it to die out soon, because:

•the expected rate of infection is less than 1•this means that on average each infected person infects less than 1 additional person

This is what This is what happened next.happened next.Were you right?Were you right?•Can you explain what happened at each stage of this epidemic from the graph on the bottom right?•The initial infection was 1 person.•A second person was infected at the next step.•There were no further infections, leaving 2 immune people and 28 uninfected people.

What is likely to What is likely to happen?happen?

Is this epidemic likely to take off?Why?

Is this epidemic likely to take off?Why?

What is likely to What is likely to happen?happen?

This epidemic is NOT likely to take off because the expected rate of infection is less than 1.

This epidemic IS likely to take off because the expected rate of infection is greater than 1.

What is likely to What is likely to happen?happen?

On average, each infected person infects less than one other person.

On average, each infected person infects more than one other person.