Qualification > Sciences
Edexcel BIOLOGY Doubts!!!!
MKh:
--- Quote from: Malak on May 02, 2012, 09:57:59 am ---How can we show variability on graph?
E.g. Unit 3B Sample assesment Q1 b (ii)
thanks
--- End quote ---
To show the variability in the data, you have to include 'range bars' or 'error bars' on each plot of your (bar or line) graph. Range bars and Error bars are the same thing. In the A2 level, you always have to include them in your graph whether asked to do so or not, but I am not sure if it's the same at AS. I assume, in the AS level also, you have to include them in your graphs since doing so usually gets you a mark.
To see an example of error bars/range bars on a bar chart, see the link below, although you don't have to know all that inorder to draw error bars/range bars on your line or bar charts. Here's the link: http://en.wikipedia.org/wiki/Error_bar
Now, for example, if you have to plot a graph of a set of mean values, you will first have to decide whether you are going to plot a bar chart or a line graph. If you have 5 or more mean values to plot, you have to draw a line graph - so plot each point and join them with a straight line of best fit or a curve, although, in most cases, you have to draw a stright line of best fit. If you have 4 or less mean values to plot, you have to draw a bar chart.
Now, how do you draw error bars/range bars on your line or bar graph?
To answer this question, ask yourself how you obtained those mean/average values that you have plotted on your line or bar graph. You have obviously used a SET OF VALUES to calculate a mean value for that particular set. This SET OF VALUES for each mean value that you have plotted, is very important as it will allow you to draw an error bar/range bar on each plot of your line or bar graph.
You have to plot a range bar/error bar with each plot on your graph (that is, with each mean value). So, for example, you have 3 mean values (23, 24 and 25) and you have to show these on a graph. Draw a bar chart (as there are 3 values to plot) and draw a bar to represent each mean value on your graph.
Now, how do you draw an error bar/range bar on each of the three bars on your graph?
So, you have calculated a mean value of 23 from a SET OF VALUES (say, 21, 22, 23, 24 and 25). One of your 3 bars on your bar chart is 23 and you calculated this value of 23 from a SET OF VALUES as mentioned above. To show the variability on your bar graph, draw an error bar/range bar on each bar. On the bar of 23, to show the error bar, draw a vertical line from the highest value (that is, 25) in your SET OF VALUES to the lowest value (that is, 21) in your SET OF VALUES and draw a small horizontal line on each end of this vertical line.
This vertical line along with a small horizontal line on each end of the line shows the SET OF VALUES that you used to calculate the plotted mean value on your (line or bar) graph. This is known as the error bar or range bar and shows the variability in the data. You have to draw such an error bar or range bar for each of your plotted value.
Although I have repeated myself on many occasions along my post here, I hope it helped your understanding of this concept. For further help, do not hesitate to quote me. You can also use google if you don't understand anything and hopefully you will get a clear explanation.
Malak:
I understand what you mean...but..
For example, In my graph for one set of values (91, 75, 84 and 69 per 30 seconds) the mean is 159.5 beats per min, now the lowest value is 69 beats per 30 seconds. To draw the error bar, should I double 69? cause it is per 30 sec and not per min?
2ndly, I have started my graph from 150 and so the lowest value in the Set of values doesnt even fit in my graph :s ? I don't think an error bar would be that long.
MKh:
--- Quote from: Malak on May 06, 2012, 11:26:50 am ---I understand what you mean...but..
For example, In my graph for one set of values (91, 75, 84 and 69 per 30 seconds) the mean is 159.5 beats per min, now the lowest value is 69 beats per 30 seconds. To draw the error bar, should I double 69? cause it is per 30 sec and not per min?
2ndly, I have started my graph from 150 and so the lowest value in the Set of values doesnt even fit in my graph :s ? I don't think an error bar would be that long.
--- End quote ---
Ohhkay, so in that case, you will have to double the lowest value (that is, 69) in your SET OF VALUES which then becomes (69*2=) 138, and the highest value (that is, 91) in your SET OF VALUES which then becomes (91*2=) 182. Now you have got your highest and lowest values in BEATS PER MINUTE since all the plots in your graph are in beats per min.
Regarding the plotting problem, can you change your scales so that your error bars can fit within your graph?
In my exam, I had to choose my scales to be able to plot my mean values as well as my error bars although I had to draw a bar graph with two bars only. So choose your scales so that your plots as well as your error bars can fit within your graph.
Also, in my exam, one of my error bars was so long that it fit the whole graph paper. Without making your scales too cumbersome, try to choose an appropriate scale so that your error bars can fit within your chosen scale, even if they are too long.
Hope this solves the problem. I will try to solve the question myself also.
rameeziiii:
Okay, massive pain in the *** question from me. HOW THE HECK DO YOU PERFORM THOSE T-TEST/Other-statistical-tests?
I just tried the one in the 2012 January paper, and understood NOTHING! I HAVE NO IDEA WHY THE VALUE OF 0.79 WAS USED INSTEAD OF THE 1.00 THE STUDENT USED, OR WHY YOU'RE SUPPOSED TO USE 7 MEANS OR ANYTHING
MKh:
--- Quote from: rameeziiii on May 08, 2012, 07:23:25 pm ---Okay, massive pain in the *** question from me. HOW THE HECK DO YOU PERFORM THOSE T-TEST/Other-statistical-tests?
I just tried the one in the 2012 January paper, and understood NOTHING! I HAVE NO IDEA WHY THE VALUE OF 0.79 WAS USED INSTEAD OF THE 1.00 THE STUDENT USED, OR WHY YOU'RE SUPPOSED TO USE 7 MEANS OR ANYTHING
--- End quote ---
Maybe try to google 't-test' or look it up on Wikipedia? Maybe the number of means has something to do with how these tests are performed?
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