Presence-absence testing versus counting
Decide which question a microbiological method was built to answer, yes or no versus how many, and match the method to the decision you have to make.
- The question decides the method: Choosing between a yes-or-no method and a counting method is impossible until you can state what decision the number is for.
- How much you tested is part of the answer: Both kinds of result are statements about a measured amount of water, so the amount has to travel with the answer.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Match the method to the decision, and keep the tested amount attached to whichever answer you report.
A treatment plant wants to know whether replacing a filter reduced the bacteria reaching the finished water. Which testing approach can answer that, and why?
Reviewed- A.A counting method before and after, because the question asks how much changed
- B.A yes-or-no method after only, since the before result no longer matters
- C.A yes-or-no method before and after, since two verdicts can be ranked against each other
- D.Either approach, because both of them detect the same group of organisms
Show the worked solution ▾
Answer: A. A counting method before and after, because the question asks how much changed
- Step 1: Read the decision the plant has to make: The word reduced is a comparison of amounts, not a check for whether anything is there.
- Step 2: Match the answer shape to the comparison: Comparing amounts requires two amounts. A verdict does not supply one.
Why it's right: Reduced is a comparison between two quantities, so the method has to return quantities on both sides of the filter change.
- B: Without the earlier figure there is no comparison at all, whichever method is used afterwards.
- C: Two verdicts of detected are identical strings. Nothing in them can be ranked as more or less.
- D: Detecting the same organisms is not the issue. The two approaches return different shapes of answer.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
Two negative results are reported for the same site. One examined 1 millilitre of water and the other examined 100 millilitres. How do the two findings compare?
Reviewed- A.They are equivalent, because both of them came back with nothing found
- B.The 1 millilitre negative is stronger, because a small amount is easier to read
- C.The 100 millilitre negative is the stronger finding of the two
- D.Neither can be compared, since a negative carries no information at all
Show the worked solution ▾
Answer: C. The 100 millilitre negative is the stronger finding of the two
- Step 1: Ask how many chances each test gave the organism: A larger amount of water contains more of whatever is in the source, so it gives the target more opportunities to show up.
- Step 2: Read the negative against those chances: Coming back empty after many chances rules out more than coming back empty after few.
Why it's right: The larger amount gives sparse organisms far more opportunity to appear, so failing to find any in that amount excludes a wider range of contamination than the small one does.
- A: Both reported nothing, and the two nothings cover very different amounts of water.
- B: Ease of reading is not what is being compared. The question is how much water each negative covers.
- D: A negative bounded by a stated amount is informative. It is an unstated amount that makes one useless.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
- A private well report prints the amount examined beside every negative, so a homeowner can see how much water the clean result covers.
Fill these in as you work through the lesson.
- Presence-absence test (returns yes or no for a set amount):
- Enumeration (returns a number per unit volume):
- Sample volume (how much water the answer covers):
- Detection limit (the least the method can register):
A presence-absence method answers 'anything in this , yes or no'. An enumeration method answers 'how many per '. You cannot convert one answer into the other after the fact.
- Why is a yes-or-no answer enough for drinking water but not for a treatment plant checking whether its filters improved?
- What does a single tube going positive tell you about how contaminated the water is?
- How does testing 100 millilitres instead of 1 millilitre change what a negative means?
A drinking water sample is run as a presence-absence test on 100 millilitres and comes back positive. Say what has been established, then say what you still cannot state about how heavily contaminated the water is, and name the method you would need to answer that.