Lecture 6: Probit Plot Flashcards
Probit analysis is
a type of regression used to analyze binomial response variables.
Binomial regression is
a technique in which the response often referred to as Y is the result of a series of Bernoulli trials
Probit analysis transforms the sigmoid dose-response curve to
a straight line that can then be analyzed by regression either through least squares or maximum likelihood.
Probit analysis can be conducted by one of three techniques:
- Using tables to estimate the probits and fitting the relationship by eye,
- Hand calculating the probits, regression coefficient, and confidence intervals, or
- Having a statistical package such as SPSS do it all for you.
Probit Analysis (PA) is commonly used in toxicology to
determine the relative toxicity of chemicals to living organisms.
How can relative toxicity of chemicals to living organisms be achieved
- Testing the response of an organism under various concentrations of each of the chemicals
- Then comparing the concentrations at which one encounters a response.
Once a regression is run researcher can use the output of the probit analysis to compare
- Amount of chemical required to create the same response in each of the various chemicals.
- Many endpoints used to compare the differing toxicities of chemicals,
- But the LC50 (liquids) or LD50 (solids) are the most widely used outcomes of the modern dose-response experiments.
The LC50/LD50 represent
the concentration (LC50) or dose (LD50) at which 50% of the population responds
Step 2 in Probit Analysis
- Take the log of the concentrations.
- This can either be done by hand if doing hand calculations or specify this action in the computer program of choice.
Step 3 in Probit Analysis
Graph the probits versus the log of the concentrations and fit a line of regression.
Step 4 in Probit Analysis
Find the LC50
