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Biology graph - dot to dot?

Discussion in 'Science' started by pookyrobin, May 21, 2010.

  1. My son plotted a graph of average number of seeds in a pod against number of plants per 0.25 square metre.
    His biology teacher said it should have been joined with a dot to dot (points were close to a horizontal line- which is what I told him to draw). In Chemistry and in teaching other sciences we have always said you don't do dot to dots (except speed-time and distance-time graphs) but either a straight line or smooth curve. What do you say? Is this sort of data an unfortunate exception, that will just confuse the pupils?
  2. My son plotted a graph of average number of seeds in a pod against number of plants per 0.25 square metre.
    His biology teacher said it should have been joined with a dot to dot (points were close to a horizontal line- which is what I told him to draw). In Chemistry and in teaching other sciences we have always said you don't do dot to dots (except speed-time and distance-time graphs) but either a straight line or smooth curve. What do you say? Is this sort of data an unfortunate exception, that will just confuse the pupils?
  3. Biology degree and MSc here and I would say a line of best fit type jobbie to see any trend (which clearly doesn't exist in this case if it's horizontal)
    there is no reason to EVER draw a dot to dot apart from say harvest in each year or something.
    phlogiston likes this.
  4. lunarita

    lunarita Established commenter

    I as a physicist always insist on line/curve of best fit, but the students have told me that their biology teacher insists on join the dots. I am unclear as to why this should be the case, the situation you describe suggests a trendline is more appropriate to me.
    Can any biologists enlighten?
  5. or something like a daily temperature fluctuation type graph.
  6. Erm, no, even as a biologist, we never 'join the dots'. Always line of best fit/curve of best fit.
    Sounds like teacher didn't know what they were talking about.
  7. This sort of graph sounds like a candidate for a histogram or bar chart - maybe someone can tell me the difference?
    Anyway, where the data represent discrete rather than continuous variables, I get pupils to draw a bar chart. NEVER dot-to-dot. The whole point of a line or curve of best fit is to be able to use the graph line between the actual plotted points to predict behaviour for a situation that was not actually measured during the experiment.
    It sounds as if the information given by reading data from the line between the plotted points on your son's graph was possibly meaningless.
  8. Institute of Biology book of Biological Nomenclature (don't have a copy at home so don't have exact quote) says that graphs should be joined dot to dot when the intermediate values are not certain. In this case the seed will not have grown at a uniform rate therefore a curve of best fit is not appropriate. With dot to dot interpolation is inappropriate; if a value is to be read from the graph a line of best fit must be drawn. We do WJEC at GCSE and A level and their graphs are nearly always dot to dot. This includes enzyme temperature graphs where any attempt to draw a curve with an optimum would be a 'guess'. Instead they would say the optimum temperature is between this and this value.
    In Chemistry, Physics and Maths graphs are a way of finding or illustrating a mathematical relationship; in most Biology experiments there are far too many uncontrolled variables for this to be possible (hence our need for staticitcal analysis). We always stress that dot to dot is for Biology and that they must do lines (curves) of best fit in the other sciences.
  9. lunarita

    lunarita Established commenter

    Interesting replies.
    I can't remember whether it was a KS3 or a GCSE student told me this, We teach as specialists, so I only teach physics and have nothing to do with the biology. It's made me want to go through the biology spec and see if there's any mention there of this type of graph.
    Monday will be soon enough ;)
  10. [​IMG]

    <font size="2" face="Arial">We use dot-to-dot a LOT, often the position of intermediate points cannot be predicted reliably.</font><font size="2" face="Arial">
    Book reference mentioned above/ link to AQA requirements... (see slide 19)
  11. peterdevon

    peterdevon New commenter

    dot to dot normally only used for time series. the example mentioned by the OP should be a line of best fit (smooth curve or straight line.)
  12. Plot a graph of number of hornets nests against science specialism...............[​IMG]
    I must admit I like the idea that it is, of course, a discrete variable (average number of peas if you cheat and say only allow whole peas) and therefore choose to do a bar chart.
    The problem I have with joining points with lines is that the line shows the extrapolated functionbetween points. If you have a straight line or a smooth curve, you can formulate a mathematical equation to fit the line. So you are saying that relationship you are examining follows that formula and you can use it to predict what the result would be at values between your points if you repeated the expt with those values.
    If you draw a dot to dot, the lines between points go all over, up and down. So the mathematical description of what is happening (assuming there is a relationship) changes constantly and we know that is not really the case. So you can't mathematically use the 'lines' to predict what happens between your points because they are not true descriptions of the relationship between the points.
    We need some joined up thinking here (and not dot to dot joined up). No wonder the pupils are confused.
  13. Oh you're such a physicist!! There are often no mathematical formulae to relate the variables in Biology unless you are doing biochemistry experiments with isolated enzymes. Graphs still show trends but seldom give meaningful mathematical relationships. E.g. you could measure your son's height every other month and plot a graph. The only known data points are those that you took. A best fit curve that misses any of the points would not be much use and attempting to read a value from the graph for an intermediate point wouldn't tell you his height on that date - it may have been constant for six weeks with a growth spurt in the two weeks before you measured. A best fit curve would give you an average growth rate but you'd miss the growth spurts in the first two years of life and at puberty.
    Actually I would have no problem with a best fit curve / line for the data he was given but suspect the Biology teacher is trying to teach what will be needed for examintions in the future.
    samlloyd73 likes this.
  14. blazer

    blazer Star commenter

    This begs the question why even plot a graph if it is dot to dot? The whole ideas of a graph is to spot a pattern. If you have to do dot to dot then there is no pattern and so a table of results (or even just a list) is good enough. A bar chart is probably taking the data too far!
  15. Physical Chemist if you must[​IMG]
    OK I can relate to that. We should be trying to sort this science wide confusion, shouldn't we? What has it said in past biology ISA (AQA) papers or other boards wrt to graphs and lines of best fit? OR do they carefully avoid any such type of data for the ISAs?
    It will make chemistry teachers' life harder if the pupils think it is acceptable to draw dot to dots.
  16. In biology papers.
  17. We do WJEC at both GCSE and A level; they like dot to dot for both (that is Biology and the Biology part of Science and Additional Science); there is one GCSE coursework exercise where they are told to draw a line of best fit because they need to read a value from the graph. We used to do AQA A level and they also expected dot to dot (again, unless using isolated enzymes e.g. changing concentration when there is direct proprtionality) but, to confuse matters at that time AQA GCSE wanted a line of best fit. I think the problem goes back to the invention of the subject science with a national curriculum that had to have the same criteria for anything and everything under that umbrella rather than the idea of the sciences which are connected by the collection of empirical evidence, fair testing (although I never used that term when I was a research scientist!) controls etc.
    Blazer - graphs do show patterns much more clearly than data tables e.g. in the measuring height example, if you don't measure the height at regular intervals you may not spot growth spurts or periods of slow growth whereas the gradient of your line will change on the graph.
  18. Tell me about it! I came into teaching after a career as a research scientist, to hear everyone going on about 'fair testing'. What! If I could ban two words it would be 'fair' and 'test'. I suppose it might have served a purpose in Junior school, but they won't get past it even at GCSE.
  19. Biologist here: best fit always for me. A saw-tooth line gives precisely read data which will always suggest a trend or pattern - unless results are truly random, inwhich case there is something wrong with the experiment. The child's height may well increase in spurts, as shown by the positions of the CROSSES (my next comment) on the graph; the line of best fit shows the trend, the rate over time.
    But no scientist should ever be joining points on a graph "dot to dot" as no point should be plotted with a dot. Where LOBF is drawn, the whole idea is that all plotted data should be shown and the line may actually pass through some of them, in which case a mere dot will be rendered invisible! So teach your pupils to plot with an x, please!
  20. My physics teacher, who claimed to be an examiner with one of the boards, taught us to plot dots with little circles round them so they couldn't be lost.
    samlloyd73 likes this.

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