[R] Heteroscedastic data due to responses at one factor level being too low to measure and assumed to be 0?

Dr. Leigh S. Sutherland l.sutherland at tecnico.ulisboa.pt
Tue Mar 1 16:01:05 CET 2016

Hello all,

I have a 3-factor experimental design.
The response is the load to break an adhesive joint.

I have a question about Heteroscedastic data ('non-uniform variability',
Google told me that's what it's called - I'm not a statistician, just an

*In essence:*

   - At one level of a factor the response values are so low to make them
   un-testable and they have to be assumed to be 0

   - However this gives no variability at that level since all responses
   are 0, (although in reality they may well have similar variability as at
   other levels).

   - This gives heteroscedastic data across the levels of the factor

   - What implications does this have for stats tests that are sensitive to
   Heteroscedastic data?

   - If I need to do anything, what can I do? can I 'superimpose' the same
   variability on level 1 results to give artificial variable values 'around'
   0kg? (this would not at all invalidate the data's validity)


Leigh Sutherland

Centre for Marine Technology and Ocean Engineering (CENTEC)
Instituto Superior Técnico
Av. Rovisco Pais
1049-001 LISBOA

Tel: +351 218 417 947

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