View Single Post
      01-30-2012, 05:45 PM   #17
ideliver
Major
ideliver's Avatar
49
Rep
1,232
Posts

 
Drives: 08 E60 M5 & 08 Infiniti QX56
Join Date: Feb 2008
Location: At the gas station

iTrader: (0)

Garage List
2006 MB S430 SOLD  [0.00]
2003 E46 M3  [0.00]
Quote:
Originally Posted by scorcherjf View Post
Okay you say your null hypothesis is the manufacturer's claim of 99.9% killing efficiency. For this you really need to have each dish tested to determine the actual percentage of bacteria that was killed. Then you take the average of the results (say 95% of bacteria killed on average), compute the standard deviation, and then you can come up with your confidence intervals. (Also if you want to be pedantic you have to first assume the results will be normally distributed.)

The problem with this is you don't really have a way to accurately measure how much bacteria is killed. If you're going to break the outcome down into 2 outcomes versus an infinite set of outcomes, then it's a different problem to consider. For 2 outcomes (either bacteria grew or it didn't) you wouldn't really be able to get a confidence interval because your outcomes aren't a continuous and infinite set (such as the percentage range between 0 and 100). You would just have to get 1000 petri dishes and if bacteria grew on 1 of them then the average killing rate would indeed be 99.9%. If bacteria grew on half of them then you would say the killing rate is 50%. This is probably not feasible for you since buying that many petri dishes is tedious.

Also, the claim that 99.9% of bacteria is killed is how much the product will kill on contact. What it doesn't guarantee is that the surviving bacteria reproduces over time if not totally eradicated. So the real test should be to get a sample, test that there is indeed bacteria in that sample, then use the product, and immediately test if the product was successful in killing the advertised rate of 99.9% of the bacteria in the sample.

Sorry about being so annoying with my answer but I don't think you can feasibly test the null hypothesis by letting the bacteria grow on a petri dish unless you constrain your outcomes to a binary set.
For this you really need to have each dish tested to determine the actual percentage of bacteria that was killed. I can do this if necessary....however, knowing what I know...nothing is going to grow after the surface is cleaned with bleach

For 2 outcomes (either bacteria grew or it didn't) you wouldn't really be able to get a confidence interval because your outcomes aren't a continuous and infinite set (such as the percentage range between 0 and 100)
I am fuzzy here, however I think a sample size calculation can be done...

So the real test should be to get a sample, test that there is indeed bacteria in that sample, then use the product, and immediately test if the product was successful in killing the advertised rate of 99.9% of the bacteria in the sample This is what we will be doing....but I will be testing some nasty stuff that will likely have bacteria and fungi...like toilet seats in public restrooms

I really appreciate the help...
__________________
Cars I've owned '76 Chevelle 350 RIP '82 Monte Carlo 350 RIP '82 Mustang 302 RIP '88 Honda CRX RIP '88 Olds Toronado sold '94 Toyota Tercel sold '99 Acura TL sold 2002 Porsche 911 sold 2003 E46 M3 Blk/gr 2006 MB S430 sold Current: 2008 Infiniti QX56 & 2008 E60 M5