Mathematics: Post your doubts here!

[Dug]

Yeah exactly. So the boundries are something like:
-0.5-9.5
9.5-19.5
19.5-34.5
Correct or not?

Class boundaries are not needed for calculating mean. We do that only in histograms and cmf curves!

 

[Scafalon40]

Class boundaries are not needed for calculating mean. We do that only in histograms and cmf curves!

You need the class boundaries to calculate the midpoint, don't you?

 

[Dug]

You need the class boundaries to calculate the midpoint, don't you?

Nope!
Class Interval | Mid-point | Frequency
0 - 9 | 4.5 | 25
10 - 19 | 14.5 | 43
20 - 34 | 27 | 91
35 - 49 | 42 | 75
50 - 69 | 59.5 | 26
70 - 99 | 84.5 | 40

Mean = (4.5)(25) + (14.5)(43) + (27)(91) + (42)(75) + (59.5)(26) + (84.5)(31) = 11270/300

 

[Scafalon40]

Nope!
Class Interval | Mid-point | Frequency
0 - 9 | 4.5 | 25
10 - 19 | 14.5 | 43
20 - 34 | 27 | 91
35 - 49 | 42 | 75
50 - 69 | 59.5 | 26
70 - 99 | 84.5 | 40

Mean = (4.5)(25) + (14.5)(43) + (27)(91) + (42)(75) + (59.5)(26) + (84.5)(31) = 11270/300

Dug I'm pretty sure you calculate the class width by using the class boundaries. Then using the class width you can calculate the midpoint! If not, could you provide me with some reference stating otherwise?

 

[Dug]

Dug I'm pretty sure you calculate the class width by using the class boundaries. Then using the class width you can calculate the midpoint! If not, could you provide me with some reference stating otherwise?

I don't have a reference but you could try calculating the mean using your method. The answer will come out wrong!

 

[PhyZac]

Interesting. Dug's method make sense. Thanks for sharing, Dug! Any thoughts, PhyZac?

Hmm, I will have do understand it first. But seems very sensible.

 

[daredevil]

Yeah exactly. So the boundries are something like:
-0.5-9.5
9.5-19.5
19.5-34.5
Correct or not?

but the marks cannot be in negative :O

 

[Scafalon40]

I don't have a reference but you could try calculating the mean using your method. The answer will come out wrong!

Actually it would be correct. You can try yourself
It's the midpoints that matter, Dug. So take the class 19.5-34.5. It's midpoint is 27. And so on. Do you have that stats book by Chambers and Cranshaw?

 

[Scafalon40]

but the marks cannot be in negative :O

Yeah I know, and that's what confuses me the most!

 

[PhyZac]

Yeah I know, and that's what confuses me the most!

For 0 it stays zero, keep it like a rule.!!

 

[Dug]

Actually it would be correct. You can try yourself
It's the midpoints that matter, Dug. So take the class 19.5-34.5. It's midpoint is 27. And so on. Do you have that stats book by Chambers and Cranshaw?

What about the mid-point of the first class 0.5 - 9.5?

 

[Scafalon40]

For 0 it stays zero, keep it like a rule.!!

Mate look: if you have two classes such as 10-19 and 20-29, then their class boundaries should be 9.5-19.5 and 19.5-29.5. That's what I was taught. I think you would concur.

 

[Scafalon40]

What about the mid-point of the first class 0.5 - 9.5?

No. I was thinking of something along the lines of -0.5-9.5. You need class boundaries! The negative confuses me, yes. The only possible reason I can think of is negative marking
On a serious note, I don't quite understand the negative part.

 

[PhyZac]

Mate look: if you have two classes such as 10-19 and 20-29, then their class boundaries should be 9.5-19.5 and 19.5-29.5. That's what I was taught. I think you would concur.

Yea I know, I got your point

But, we cant have -ve number in some cases (many cases) so consider 0!

 

[Scafalon40]

Yea I know, I got your point

But, we cant have -ve number in some cases (many cases) so consider 0!

So you support Dug's theory that class boundaries aren't required when calculating midpoints? I am considering 0. It's driving me mental!

 

[PhyZac]

So you support Dug's theory that class boundaries aren't required when calculating midpoints? I am considering 0. It's driving me mental!

Nope, I don't support that, I agree to your point in that case.

But I think this will satisfy you
http://www.examsolutions.net/maths-revision/statistics/representing-data/histograms/tutorial-1.php

 

[daredevil]

For 0 it stays zero, keep it like a rule.!!

Yeah I know, and that's what confuses me the most!

u know about these class boundaries and stuff our sir told us that we can either subtract and add respectively to the 1st and the second value or we cud just add 1 to the second one or subtract one from the first one... just have to make a difference with 1. and given this question i dont think there's much we can do about the negative value so its better to add 1 to the second one.

btw the answers to these class widths are ambigous because they are different in almost every marking scheme. just make ur best judgement according to the question is pretty much all u can do.
also if u take 10.5 till 20.5 in this specific example under discussion u will still get a decimal value for the mean (15.5) ryt?

 

[Scafalon40]

Nope, I don't support that, I agree to your point in that case.

But I think this will satisfy you
http://www.examsolutions.net/maths-revision/statistics/representing-data/histograms/tutorial-1.php

Youtube doesn't work here mate
I have all these examsolutions videos downloaded, though. It's gonna take me some time to find it. Hold on!

 

[Scafalon40]

u know about these class boundaries and stuff our sir told us that we can either subtract and add respectively to the 1st and the second value or we cud just add 1 to the second one or subtract one from the first one... just have to make a difference with 1. and given this question i dont think there's much we can do about the negative value so its better to add 1 to the second one.

btw the answers to these class widths are ambigous because they are different in almost every marking scheme. just make ur best judgement according to the question is pretty much all u can do.
also if u take 10.5 till 20.5 in this specific example under discussion u will still get a decimal value for the mean (15.5) ryt?

Not 10.5, 9.5!
And you're right about these different markschemes. I guess there's nothing we can do....
Btw if you could just provide me with some sort of link, some reference etc I'd be quite satisfied
Thanks anyway

 

[Scafalon40]

Nope, I don't support that, I agree to your point in that case.

But I think this will satisfy you
http://www.examsolutions.net/maths-revision/statistics/representing-data/histograms/tutorial-1.php

I've watched the video. It still doesn't make sense. If you take the class boundaries as 0 and 9.5, you'll get the midpoint as 4.75, whereas the mark scheme clearly states it as 4.5. That's what I don't understand!!!!!!!!!!!!!!!!!!!!!!!!!