First Python lab

0. Honesty rule (read first)

You may ask a tool to explain a syntax error. You may not submit code you cannot explain line by line. Mastery Friday is closed-notes on the idea, not the brand of editor.

1. Run or trace this

counts = [12, 15, 14, 18, 16]  # yeast colonies, sample plate
total = 0
for n in counts:
    total = total + n
mean = total / len(counts)
print("n =", len(counts))
print("mean =", mean)

If you do not have Python installed, trace it on paper. What prints? Why divide by len(counts)?

2. Change one variable

Add a sixth count of 11. Recalculate. Write: independent idea (another trial) vs the statistic (mean). That is experimental design language Biology will keep using.

3. CER, not a screenshot

Claim about the mean. Evidence = the list. Reasoning = what a mean hides (an outlier of 40 would drag it). Put this in the lab notebook — that is transcript evidence.

4. AI check (5 min)

Paste the code into a chatbot and ask what it does. Then close it. Explain the loop out loud without looking. If you cannot, the tool learned it — you didn’t.