Lecture 4: Experimental Design Flashcards
What are the 3 ways we quantify ecology/measure interactions between organisms and their environment?
- formulate questions and hypotheses about observations
- design experiments and collect data to test hypotheses
- analyze data and interpret results in order to draw conclusions
What are the 8 components of experimental design?
- independent variable
- dependent variable
- groups in the experiment (treatment vs control)
- standardized variables
- replication
- data collection
- statistical analysis
- conclusion
What is the independent variable?
the variable that is intentionally different among the experimental groups
It is the variable that is manipulated by the researcher
what is the dependent variable?
the variable that is measured by the researcher
it is the one we expect to be affected by the changes the researcher makes
What is the first step in an experimental design?
making observations
What is an example of an observation?
most green frogs, Rana clamitans, reproduce in early march
What is the second step in an experimental design?
Formulating a scientific question
What are the 2 types of scientific questions in an ecological experimental design?
- proximate questions
2. ultimate questions
Describe proximate questions
They ask “HOW does it happen?”
What is causing this to happen?
They are based on immediate cause
the physiological, genetic, hormonal, motor responses to environmental stimulus
Describe ultimate questions
they ask “WHY does it happen?”
Considers evolutionary explanations; relationship to fitness (survival or reproductive success)
Give an example of a PROXIMATE question for the observation that most green frogs reproduce in early March
Does the warming water temperature in early March affect the timing of frog reproduction?
Give an example of a ULTIMATE question for the observation that most green frogs reproduce in early March
Does the warming water temperature in early March affect reproductive success in frogs?
Define null hypothesis
Statement that the independent variable will not have an effect on the dependent variable
Define alternative hypothesis
Statement that the independent variable will have an effect on the dependent variable
Which hypothesis is tested in an experimental design? Why?
the NULL hypothesis
We always assume the null is true until we have statistically significant data that allows us to reject the null hypothesis
By testing the null, it avoids introducing biases into the experiments
Which is the INDEPENDENT variable in this null hypothesis example: Water temperature does not have an effect on the reproductive success of green frogs
Water temperature
it is what can be manipulated
T or F: in all cases you can manipulate your independent variable for your experiment
FALSE.
Sometimes you can, other times you have to work with the natural variation
Which is the DEPENDENT variable in this null hypothesis example: Water temperature does not have an effect on the reproductive success of green frogs
the reproductive success
it is what will be measured
T or F: this null hypothesis: “Water temperature does not have an effect on the reproductive success of green frogs” example provides enough detail about the dependent variable for the experimental design?
FALSE.
reproductive success (dependent variable) can be measured in a lot of ways, this statement is not detailed enough
What are 4 ways to measure reproductive success in frogs?
- egg production
- fertilization success
- hatching success
- offspring survival
What else needs to be considered when deciding how to measure a dependent variable?
the difficulty of collect the data and how useful it will be in answering the question
ex. later life stages are harder to measure, but better estimators of reproductive success
What 4 elements does a formal hypothesis include?
- independent variable
- dependent variable
- units of measurement for both ^^
- the scientific name of the study species
Give a more formal null hypothesis for the example of the green frogs and water temperature
Water temperature (degrees C), does not have an affect on egg production (#eggs/egg mass) of green frogs, Rana clamitans.
Give a more formal alternative hypothesis for the example of the green frogs and water temperature
Water temperature (degrees C), does have an affect on egg production (#eggs/egg mass) of green frogs, Rana clamitans.
After making the hypotheses, what comes next?
You have to make a prediction of the results you would expect to see if the ALTERNATIVE HYPOTHESIS IS SUPPORTED
Give an example of a prediction for the green frogs and water temperature hypotheses
If water temperature (degrees C), does have an affect on egg production (#eggs/egg mass) of green frogs, Rana clamitans, THEN
the number of eggs produced per egg mass should be positively correlated with water temperature
Describe positive correlation
one variable increases as the other increases
Describe negative correlation
One variable decreases as the other increases
T or F: when describing your experiment, you need to explain how you will collect the data so that someone else can replicate your experiment
TRUE
What are the 3 types of experiments in ecology?
- Natural/observational experiments
- manipulative experiments
- mathematical model
Describe natural experiments
rely on natural variation in the environment so that researchers do not manipulate groups in the experiment
Give 4 reasons why we might use a natural experiment
- working with endangered species (cannot manipulate environment, collect offspring)
- working in parks (organisms are protected against manipulation)
- Some manipulations would harm the organisms
- Always do this as a first step to ensure a relationship actually exists
Why are natural experiments not always the best choice?
Because it is difficult to isolate the exact cause and affect
Describe manipulative experiments
test hypotheses by artificially changing the factor that is hypothesized to be the underlying cause of a phenomenon
- essentially applying treatments to test groups
Control groups are usually required
Why are control groups required in addition to treatment groups for manipulative experiments?
to demonstrate that the manipulation did not have any unintended effects on the results
Give 3 reasons to do a manipulative experiment
- to isolate specific variables to determine cause and effect
- natural variation with population is too low
- there is no barrier to doing so, there is much more power in the results
Describe mathematical models and give an example
represent a system with a set of equations representing hypothesized relationships between components of the system
- write a model predicting the hatching success in frogs given different values for several environmental conditions
- then measure conditions and hatch success in the field to verify the accuracy of the model
When are mathematical models useful?
- to answer theoretical questions
2. to predict outcomes in response to multiple scenarios
What are the disadvantages of natural experiments?
difficult to isolate variables
correlation does not equal causation
What are the disadvantages of manipulative experiments?
creates simplified environments with potential of lab effects (ie, testing outside of natural conditions can remove confounding variables)
What are the disadvantages of mathematical modelling?
it introduces assumptions that may or may not be correct
T or F: it is best to pick one type of experiment
False. They all have their limitations so it is best to use multiple approaches
From this example:
“Observe frog reproduction from and measure egg masses daily from first to last date of reproduction. Collect all new egg masses daily and record water temperature at time and location of collection. Gently spread each egg mass out on a tray and count and record number of eggs. Return all egg masses to original location in pond immediately after counting.”
Which type of experiment was done? how can you tell?
Natural because the dependent variable (water temperature) was not altered
What is the purpose of scatterplots to show results?
They show the correlation between 2 variables
if the dots are centred around a non-horizontal line, the variables are correlated
if the line is horizontal and the dots are everywhere, there is no correlation
On a scatterplot, what does the ‘r’ value represent?
r value = correlation
r = 0 = no correlation = horizontal graph
r = +1 = positive correlation = line with positive slope
r = -1 = negative correlation = line with negative slope
On a scatterplot, what does the ‘p’ value represent?
significance levels
significance = 0.05
less than 0.05 = statistically significant results
more than 0.05 = no statistically significant effect
What is the hypothetico-deductive/scientific method? What are the steps?
- question
- form hypotheses
- prediction
- test hypotheses
- if you fail to reject the null hypothesis - look at the experiment and see if there are problems
- if you reject the null hypothesis, formulate other questions that could further the research
