1. Center for Vector Biology
Catch the Fever!
Calculation of Minimum Field Infection Rates (MFIR) and Maximum Likelihood Estimate (MLE)
Lisa Reed, Ph.D
Center for Vector Biology
Rutgers University
21 April 2011

2. What is in this talk? What is an MFIR? History
How do we calculate MFIR?
Getting data from ESRI
Formulas and Pivot Table in Excel
Calculating MFIR
What is MLE
Using Excel add-in to calculate MLEs
Demos
Which is best?
AP photo

3. Minimum Field Infection Rate
Female mosquitoes are live-collected from traps, and grouped by species into vials
Each vial may contain up to a certain number of mosquitoes
50 or 75
100
Pools are evaluated for virus (RT-PCR) and given a positive or negative result.
Positive pools are counted, divided by all mosquitoes making up all pools evaluated and multiplied by 1000
Total number of positive pools
Total Number of mosquitoes (positive and negative pools)
*1000

4. Number of infected mosquitoes per 1000
It represents the MINIMUM number
Each pool acts like a single infected mosquito
A pool can have 30 out of 50 infected but will still only contribute 1 to the rate
=66.67
=66.67
=166.67
=33.34

5. MFIR History First mention in Proceedings of MFIR values came in 1974
Complicated process to determine arbovirus presence
Grown in culture
Cell culture (Vero)
Plaque Assay
Baby mice, chicks or hamsters
Tested using hemagglutination, fluorescent antibody
Not computerized
Our understanding of the epidemiology of EEE and other arboviruses were becoming known.

6. Getting Your Data Download data from Department of Health site.
Use the correct variables
Put the data in the correct form
Using formulas in Excel
Use pivot tables in Excel
Apply the formula for MFIR

7. Getting Data from the Health Department
Go to and log in.
Click on “West Nile Virus Surveillance”
Go To Mosquito>Reports and Maps>Extract Mosquito Data

8. Getting Data
Select the appropriate dates

9. Picking Columns is Critical
Lots of columns to choose.
Choosing ALL of them can create a HUGE and possibly incomplete data file.
There are certain columns you need for calculating infection rates (16):
“First 6”
USI, SUBUSI, PHELMPID, DATECOLL, SPECIES, MOSCOUNT
RESULT, SITENAME, SURVTYPE, LAT, LON, COUNTY, FINSTAT, EEEFINAL, LABNAME, PATHOGEN

10. Extracted Data

11. Save “Data Dump” file

12. Saving to CSV file

13. In Excel

14. Format/Calculate Variables
Formulas:
A formula in Excel starts with = or + sign
A formula in Excel refers to another variable (generally)
Week Number formula:
+weeknum(date)
Pull on bottom corner
to fill column with formula
(double click)
Substitute Aedes for
Ochlerotatus

15. Preserving Your Changes
Copy the column with the formula (highlight and right-mouse click)
Paste as “Values”
Hit Save

16. Using Pivot Tables to Get the Information You Want
Pivot Tables Sorts and Manipulates Data in a Summary Fashion
Uses a Layout Wizard to Make Your Choices Easier

17. Pivot Table 1
Highlight Data (hold down shift key and arrow down, then arrow across)
INSERT Pivot Table

18. Pivot 2 A new sheet is created Can be modified
Lists variables in your dataset
Has place for column, row, value and filter
What do these mean?

19. Drag Variables to Label, Value and Filters
Drag Species to Row Labels and it will place the species names, one to each row
Drag Results to Column Label
Drag Pathogen to Filter
Drag Moscount to Values

20. Drag Moscount to Values twice
We need both the total number (sums) of mosquitoes tested AND number of pools (counts)
Change the values by clicking on moscount down arrow to change value to sums

22. Copy Pivot Table to New Sheet
Use “Paste Special”
Use Values

23. Get Rid of Unnecessary Columns
Click on column header and control key to select columns
Delete

24. Calculate MFIR Add formula to adjacent column
(Pos pools)/(Total Mosquitoes) * 1000
Fill in formula to end, format if you wish

25. Your MFIR values
You can add different variables (such as type of trap, months, township) if you are interested in more precise information
You can use data from different time periods
You can use this data to justify your actions
DEMO

26. MLE History Maximum Likelihood Estimate
Statistical term that takes into account POOL SIZE
Not easy to hand calculate
Download Excel add-in from CDC:
Calculates MLE, an adjusted MLE, MFIR and confidence intervals relevant to MLE.
MLEs are more accurate when the infection rate is higher or when pool size is large because there is more of a likelihood that a pool will have more than one infected mosquito.

27. Adding an Add-in to Excel

28. Open a downloaded dataset
Find your variables
Sort on Result – delete zero rows.
Add and column next to Result
Change “Result” to a number using a formula:
+if((result)= “Neg”,0,1)

29. The Add-in

31. Caution!
Sort on the results column and delete any values not Neg or Pos.
Sort on pathogen column and delete any pathogens not the one you want to look at.

32. The Result on a New Sheet
Demo

33. Center for Vector Biology
Questions?
Lisa Reed, Ph.D
Center for Vector Biology
Rutgers University
21 April 2011