<Tr> <Td> Event rate (ER) </Td> <Td> EER = EE / ES = 0.1, or 10% </Td> <Td> CER = CE / CS = 0.4, or 40% </Td> <Td> </Td> </Tr> <Table> <Tr> <Th> Equation </Th> <Th> Variable </Th> <Th> Abbr . </Th> <Th> Value </Th> </Tr> <Tr> <Td> CER - EER </Td> <Td> absolute risk reduction </Td> <Td> ARR </Td> <Td> 0.3, or 30% </Td> </Tr> <Tr> <Td> (CER - EER) / CER </Td> <Td> relative risk reduction </Td> <Td> RRR </Td> <Td> 0.75, or 75% </Td> </Tr> <Tr> <Td> 1 / (CER − EER) </Td> <Td> number needed to treat </Td> <Td> NNT </Td> <Td> 3.33 </Td> </Tr> <Tr> <Td> EER / CER </Td> <Td> risk ratio </Td> <Td> RR </Td> <Td> 0.25 </Td> </Tr> <Tr> <Td> (EE / EN) / (CE / CN) </Td> <Td> odds ratio </Td> <Td> OR </Td> <Td> 0.167 </Td> </Tr> <Tr> <Td> (CER - EER) / CER </Td> <Td> preventable fraction among the unexposed </Td> <Td> PFu </Td> <Td> 0.75 </Td> </Tr> </Table> <Tr> <Th> Equation </Th> <Th> Variable </Th> <Th> Abbr . </Th> <Th> Value </Th> </Tr> <Tr> <Td> CER - EER </Td> <Td> absolute risk reduction </Td> <Td> ARR </Td> <Td> 0.3, or 30% </Td> </Tr>

What is the null value for a risk difference