Competent cells competent students

At the Institute, I have a bright and engaged class for Molecular Biology.  This last week we were making "competent cells" and needed to make a batch of agar plates (as we professionals casually refer to petri-dishes) which, as well as the agar to make it set and sufficient nutrients to grow bacteria, needed the precise addition of three other reagents: IPTC, X-gal and Ampicillin.  After 2 and a half years of training and practice, I feel I can let them off to read the label on a container of agar, weigh out the required amount to a suitable tolerance, add the water and boil it all up.  I've been making it explicit that I believe they are more technically competent than me - they could hardly be less - and it's true that about half of them really are. But more important than technical competence is training them to think clearly, make accurate and reliable/repeatable calculations stand by their own judgment.  They will after all be out in the work-force in a couple of years time and will have to stand up and correct the sales-manager when he makes a gross error in his predictions of growth . . . or be saddled with a year of working nights and weekends to achieve the target.

We only have a couple of weighing machines in each lab and it would take forever for each pair of students to weigh out each of a half dozen reagents at the beginning of each class, so standard practice is to ask the technician to make up some concentrated stock solutions and have everyone add the correct amount of this to a 100ml bottle of agar after the latter has been boiled to sterilise it.  The agar is then poured into the plates which are then ready to grow (or not grow - it's a selective regime) the bugs. In baby class we tell the students how much of each stock solution to add, later on we expect everyone to be able to make the calculations themselves.

I have the last section of the week and my colleagues told me they had spent a good part of the class explaining how to do the calculations. I wasn't having any of that.  There are at least a couple of conceptually different ways to doing these dilution calculations and I didn't want to steam-roller my way over the whole group, really confusing those who might have nailed another way into their heads.  So I gave everyone a little docket and ten minutes to do the calculations in whatever way seemed good.  At the end of that I gathered up the attempts anonymously (it's a good thing to know that you've gotten it wrong but it's not-so-good to have everyone else know you got it wrong) and collated the results:

Person	IPTC	X-gal	Ampic	Person	IPTC	X-gal	Ampic
Alice	0.25	0.4	NA	Fred	0.004	0.002	0.001
BobII	0.25	0.4	1.0	Ger	250	0.4	1
Chuck	250	400	1000	Harry	250	400	1000
Delia	250	0.4	1000	Irene	250	400	1
Ernest	0.25	0.4	1	Jack	0.2	0.4	1000
Shy	NA	NA	NA	Wan	NA	NA	NA

Only two of the answers are precisely the same, and it is some consolation that those two are technically 'correct'. Most of the others are practically correct: when they actually had to take aliquots of the stock they would have noticed that they were out by 1000 across the board.  Others are effectively correct: it wouldn't affect the results if they added 200 rather than 250. On my watch the only answers that are wrong are those where no attempt has been made.  I then showed them the way that I ("a bear of little brain") approach these calculations, exhorting them to please point out if I made a sales-manager error.  I try to make that easier by insisting that I am a danger and a liability in any lab and that they have much more experience (hours spend in a white coat this year) than I do.

I then let them off to start doing the experiment and while the autoclave was sterilising the agar, they went off for tea. They are trying to persuade the two French exchange students that tea is fit to drink.  When they came back - tea really does sharpen the brain, French people - they confessed that they/we would have to dump their batches of agar into the trash.  Turned out that Fred (or Delia or Ger) had taken LB broth off the reagent shelf rather than LB agar and that everyone else had weighed their stuff out of the same jar without stopping to read the label properly.  They'll never make that mistake again! The clueless are slavish.  But if I had hovered over them as they did their work they would never have learned from the mistake . . . and it is entirely likely that I wouldn't have notice that mistake myself.