SCIE 211 Lab 4: Correlation and Causation
Instructions
Introduction
Identifying the extent and type of relationships between variables (or phenomena, events, traits, etc.) is an important aspect of the scientific method. The term correlation is used to refer a mutual relationship that is thought to exist between any two variables, phenomena, events, etc. While correlation plays an underlying role in establishing causation (i.e. the events must be related in order for A to cause B), the existence of a relationship between variables is not sufficient to imply causation.
The process of successfully attributing causation is difficult; the causal chain (or pathway) of events is often not clear. Can we be sure that A causes B, or is it, in fact, the other way around that B causes A? Or, is there a hidden third, extraneous, or confounding factor C that can cause one or the other or both? This hidden or lurking third factor (another variable at play/an alternative explanation) is called the extraneous, spurious, or confounding variable.
There are three criteria for figuring out whether or not there is evidence for causation:
1) There exists a strong and consistent correlation. This means that when the alleged cause A is present, the alleged effect B tends to be present as well. Also, there should be a plausible explanatory model (that is consistent with the data and fits with other scientific understanding) so we can explain the correlation.
2) There is precedence. This means that both the time-order and direction have been established. In other words, to say that A causes B, the cause A must come before effect B —or that B does not happen unless A occurs first.
3) All other confounding factors (lurking, spurious, extraneous, or third variables) or alternative explanations have been ruled out. Causation can only be established if A has been shown to directly cause B, without any other intervening variables. This allows us to make predictions in advance, that A will cause B. This predicative relationship can be seen, for example, in the dose relationship — the larger the dosage, the stronger the response.
Correlation and Causation
To demonstrate this principle, you will look at the relationship between the diameter of a balloon and lung volumes.
Tidal volume is the volume of air that you move into and out of your lungs each time you breathe normally. Average tidal volume is roughly 500 mL (Moini, 2020)
The amount of air that can be inspired forcibly beyond tidal volume is called the inspiratory reserve volume (IRV). The average inspiratory reserve volume varies dramatically with sex: 3000 mL in males, 2100 mL in females (Moini, 2020).
The amount of air that can be expelled forcibly beyond tidal volume is called the expiratory reserve volume (ERV). Just like the inspiratory reserve volume, the expiratory reserve volume also varies dramatically with sex: 1100 mL in males, 800 mL in females (Moini, 2020).
Vital capacity represents the maximum amount of exchangeable air that our lungs can move. It is the sum of the tidal volume, inspiratory reserve volume, and expiratory reserve volume, or:
Vital capacity (mL) = Tidal volume (mL) + IRV (mL) + ERV (mL)
In this laboratory, you will use a balloon to measure your tidal volume (volume of air moved in normal breath) and vital capacity (maximum volume of air moved). You will consider lurking variables/confounding factors that contributed to any deviation from expected values.
Correlation and Causation
Objectives:
After completing this laboratory, you should be able to:
- Discuss the danger of confusing correlation with causation.
- Identify extraneous or confounding factors.
- Explain how we determine support for causation.
- Evaluate causal claims.
Materials:
- 1 Balloon (12 inch)
- Metric Ruler
- 1 binder clip (or fingers)
- Pencil
- Calculator
Procedure – Part I: Measuring Tidal Volume
- Blow up and deflate the balloon several times to stretch it out.,
- Sit down and relax. Inhale normally and then exhale only that of a normal breath into the balloon.,
- Immediately twist and clamp the balloon (or pinch with your fingers) so that no air escapes.,
- Place the tip of a pencil vertically onto the zero cm mark on the ruler. Place the balloon on its side next to the pencil. (See figure 1).,
- Holding the balloon in position, move your pencil to the other side of the balloon.,
- Record the diameter (cm) in the table on the Lab 4 Worksheet. Make sure your recorded value has the correct number of significant figures.,
- Repeat the entire process 2 more times.,
- Calculate the average balloon diameter for your three trials on the Lab 4 Worksheet , Figure 1
Procedure – Part II: Measuring Vital Capacity
- Use the same balloon as used in Part I.
- Inhale as deeply as you can and exhale as much air into the balloon as you can.
- Immediately twist and clamp the balloon (or pinch with your fingers) so that no air escapes.
- Place the tip of a pencil vertically onto the zero cm mark on the ruler. Place the balloon on its side next to the pencil. (See figure 1).
- Holding the balloon in position, move your pencil to the other side of the balloon.
- Record the diameter (cm) in the table on the Lab 4 Worksheet. Make sure your recorded value has the correct number of significant figures.
- Repeat the entire process 2 more times.
- Calculate the average balloon diameter for your three trials on the Lab 4 Worksheet.
Analysis
- Use the graph in Figure 2 (below) to estimate the average diameters of your balloon filled by a normal breath and the balloon filled by a deep breath into cc (cubic centimeters) of lung volume. Record the values in the table on the Lab 4 Worksheet.
- Using the conversion 1 cc = 1 mL, convert the volumes in cc to volumes in mL. Record these values in the table on the Lab 4 Worksheet. These represent your observed tidal volume and observed vital capacity.
- Record the expected values (from pre-lab questions 1 an 2) in the table on the Lab 4 Worksheet.
- Calculate and record the % difference. Remember, the % difference is:
Correlation and Causation
% difference = |(Observed volume – Expected volume) x 100%
Expected volume
Figure 2: Relationship Between Balloon Diameter and Volume of Exhaled Air
Submit your completed Lab 4 Worksheet to the drop box by the due date indicated for the assignment.
References
Moini, J. (2020). Anatomy and Physiology for Health Professionals, Third Edition. Jones and Bartlett Learning.
Measuring Lung Capacity (n.d.). Retrieved from http://www.biologycorner.com/worksheets/lungcapacity.html