Why does the ecdf function return duplicate x values?

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Alex Knight
Alex Knight 2014-9-24
回答： Huiru Li 2020-4-9
When calling the ecdf function (empirical cumulative distribution function), why is the first "x" value always duplicated? This means that the first 'F' output is always 0. For example:
Trial>> y = 0:10;
Trial>> [f,x] = ecdf( y );
Trial>> disp(x')
0 0 1 2 3 4 5 6 7 8 9 10
Trial>> y = 5:15;
Trial>> [f,x] = ecdf( y );
Trial>> disp(x')
5 5 6 7 8 9 10 11 12 13 14 15
In other words, the minimum value in the input array always appears twice, but the corresponding F value in the first instance is always 0. Since the ECDF is given by:
...is this correct?
Thanks!

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采纳的回答

Tom Lane
Tom Lane 2014-9-24
This is just for convenience in plotting. Compare these:
>> y = rand(10,1);
>> [f,x] = ecdf(y);
>> subplot(2,1,1); stairs(x,f)
>> subplot(2,1,2); stairs(x(2:end),f(2:end))
Your formula is correct. If you need only the values after each jump in the cdf, you can discard the first value from both x and f.

更多回答（1 个）

Huiru Li
Huiru Li 2020-4-9
y = rand(10,1);
>> [f,x] = ecdf(y);
>> subplot(2,1,1); stairs(x,f)
>> subplot(2,1,2); stairs(x(2:end),f(2:end))

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