3. An investigator, performing vegetable annn an animal study designed to evalua

Question

3. An investigator, performing vegetable annn an animal study designed to evaluate the mary carcinogen- animal esis risk, randomly assigned f groups (control diet, control dict plus vegetable mixture plus vegetable mixture, 2; control diet plus vegetable- and control diet plus vegetable-fiber mixture, 2). Mammary emale Sprague-Dawley rats to five dietary 1; control dict 1: iber mixture, umors were ed by a single oral dose (5 mg dissolved in 1.0 ml. corn oil) of 7,12 2-dimethylbenzía )anthracene (DMBA) administered by intragastric intubation, i.e., the starting point for this study is when DMBA was given. Starting 6 weeks after DMBA administration, each rat was examined once weekly for 14 weeks (post DMBA administration) and the time in days) until onset of the first palpable tumor was recorded. We wish to make an inference about the marginal distribution of the time until a tumor is detected. Describe, in detail, the types of censoring that are represented by the following rats. (a) A rat who had a palpable tumor at the first examination at 6 weeks after intubation with DMBA (b) A rat that survived the study without having any tumors. (c) A rat which did not have a tumor at week 12 but which had a tumor at week 13 after inturbation with DMBA (d) A rat which died (without tumor present and death was unrelated to the occurrence of cancer) at day 37 after intubation with DMBA. (e) Confining our attention to the four rats described above, write down the likelihood for this portion of the study

Explanation / Answer

Censoring is the usage of defined sets of methods for analysing survival data where the outcome/occurance of the event of interest is variable with time. There are 3 main types of censoring

1)Right censoring: This type of censoring takes place when the expected results occur after the event or the experiment has ended.

2)Left censoring: This type of censoring takes place when the expected results occur before the event or the experiment has begun and the resultant death is often due to related to non-experimental causes.

3)Interval censoring: This type of censoring takes place when the expected results occur within the given interval of the event or the experiment however the exact time is unknown.

a) This will be considered as Interval censoring as the event of the occurance of the tumor has taken place exactly on the 6th week which lies within the given time intervel for the occurance of the event.

b)This will be considered Right censoring because as the event of the occurance of the tumor has not taken place after 14 weeks which exceeds the given time intervel assigned for the occurance of the event.

c)This will be considered as Interval censoring as the event of the occurance of the tumor has taken place exactly on the 13th week which lies within the time intervel assigned for the occurance of the event.

d)This will be considered as Left censoring as the event of the occurance of the tumor has not taken palce however the subject has died before the time intervel assigned for the occurance of the event from the causes unrelated to the experiment.

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