[R] rlnorm behaviour

[Thierry Onkelinx]

Please keep r-help in cc.

Yes. Have a look at this example

ifelse(
  sample(c(TRUE, FALSE), size = 0.5 * length(letters), replace = TRUE),
  letters,
  LETTERS
)


ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey

2016-06-14 17:31 GMT+02:00 Ayyappa Chaturvedula <ayyappach op gmail.com>:

> Thank you very much for your kind support.  The length of my condition
> vector is ~80 because I want only Sex==1 and else will be the other.  I
> understand now how ifelse works.  If the vector of the simulated vector is
> longer than the condition vector, then it takes the first few elements to
> match the length of condition vector and discards the rest?
>
> Regards,
> Ayyappa
>
> On Tue, Jun 14, 2016 at 10:15 AM, Thierry Onkelinx <
> thierry.onkelinx op inbo.be> wrote:
>
>> Dear Ayyappa,
>>
>> ifelse works on a vector. See the example below.
>>
>> ifelse(
>>   sample(c(TRUE, FALSE), size = length(letters), replace = TRUE),
>>   letters,
>>   LETTERS
>> )
>>
>> However, note that it will recycle short vectors when they are not of
>> equal length.
>>
>> ifelse(
>>   sample(c(TRUE, FALSE), size = 2 * length(letters), replace = TRUE),
>>   letters,
>>   LETTERS
>> )
>>
>> In your code the length of the condition vector is 200, the length of the
>> two other vectors is 100.
>>
>> Best regards,
>>
>> ir. Thierry Onkelinx
>> Instituut voor natuur- en bosonderzoek / Research Institute for Nature
>> and Forest
>> team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
>> Kliniekstraat 25
>> 1070 Anderlecht
>> Belgium
>>
>> To call in the statistician after the experiment is done may be no more
>> than asking him to perform a post-mortem examination: he may be able to say
>> what the experiment died of. ~ Sir Ronald Aylmer Fisher
>> The plural of anecdote is not data. ~ Roger Brinner
>> The combination of some data and an aching desire for an answer does not
>> ensure that a reasonable answer can be extracted from a given body of data.
>> ~ John Tukey
>>
>> 2016-06-14 17:02 GMT+02:00 Ayyappa Chaturvedula <ayyappach op gmail.com>:
>>
>>> Dear Group,
>>>
>>> I am trying to simulate a dataset with 200 individuals with random
>>> assignment of Sex (1,0) and Weight from lognormal distribution specific
>>> to
>>> Sex.  I am intrigued by the behavior of rlnorm function to impute a value
>>> of Weight from the specified distribution.  Here is the code:
>>> ID<-1:200
>>> Sex<-sample(c(0,1),200,replace=T,prob=c(0.4,0.6))
>>> fulldata<-data.frame(ID,Sex)
>>> fulldata$Wt<-ifelse(fulldata$Sex==1,rlnorm(100, meanlog = log(85.1),
>>> sdlog
>>> = sqrt(0.0329)),
>>>                     rlnorm(100, meanlog = log(73), sdlog = sqrt(0.0442)))
>>>
>>> mean(fulldata$Wt[fulldata$Sex==0]);to check the mean is close to 73
>>> mean(fulldata$Wt[fulldata$Sex==1]);to check the mean is close to 85
>>>
>>> I see that the number of simulated values has an effect on the mean
>>> calculated after imputation. That is, the code rlnorm(100, meanlog =
>>> log(73), sdlog = sqrt(0.0442)) gives much better match compared to
>>> rlnorm(1, meanlog = log(73), sdlog = sqrt(0.0442)) in ifelse statement in
>>> the code above.
>>>
>>> My understanding is that ifelse will be imputing only one value where the
>>> condition is met as specified.  I appreciate your insights on the
>>> behavior
>>> for better performance of increasing sample number.  I appreciate your
>>> comments.
>>>
>>> Regards,
>>> Ayyappa
>>>
>>>         [[alternative HTML version deleted]]
>>>
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>>
>>
>

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