Enhancing Thinking Skills in Science Context

Lesson 2Causal Inference –Mill’s Methods of causal reasoning

Causes Effects

Inductive reasoning

Causal relationships among events

Form 5 groups

Study and discuss the given cases

Group presentation

Activity 1a

Disease X was recorded long time ago. People with disease X would have shaking chills at regular time intervals (usually at mid-day between 11 a.m. to 12 noon), followed by high fever lasting for 2 to 6 hours. The patients may also have symptoms like headaches, vomiting, delirium, anxiety and restlessness. It was also discovered that the blood of the infected patients contained a micro-organism that was a new species of parasite at that time.

Activity 1a

Some doctors# also observed that people sleeping outdoors were more susceptible to get the disease than those sleeping indoors; people sleeping near a camp fire were less susceptible to get the disease.

Activity 1a

A doctor (Dr. R) made a lot of efforts in the research of disease X. He observed that Disease X was widely spread in the following areas in 1895:

Activity 1a

Place Natural Environment

A Near a river

B Inside a forest

C Near the marshes

D Paddy fields

Activity 1a

Dr. R also observed that there were a lot of mosquitoes in places A, B, C and D. He suggested that mosquitoes might be responsible for the transmission of the micro-organisms that caused Disease X.

Task 1a:

1. Use flow charts / comparison tables / diagrams to illustrate how Dr. R arrived at his proposition that mosquitoes might be responsible for the transmission of the micro-organisms that cause disease X.

Task 1a:

2. Use simple diagrams to show the logic of the causal relationship in case 1(a).

Activity 1b

In an uneventful afternoon, four students of school A suffered from severe diarrhea. All of them went to the hospital and the doctor naturally suspected that the symptom might be related to the food eaten by the students during lunch.

Activity 1b

The doctor asked the students what they ate for lunch and their answers were as follows:

Student Food items

1 pizza, orange juice, cookie, vegetable salad

2 hot dog, French fries, vegetable salad, iced tea

3 pizza, vegetable salad, coca cola, cheese cake

4 hamburger, orange juice, vegetable salad, cheese cake

Task 1b:

1.Use a table / diagram to show the logic of the possible cause of diarrhea among the students.2.Do you know the name of this method of establishing causal relationship?3.What is the precaution of applying the method (in (2)) in causal reasoning?

Task 1b:

Case/event Circumstance Effect 1 B,C, D, A X

2 E, F, A, G X

3 B, A, H, I X

4 J, C, A, I X

Task 1b:

Mill’s Method of Agreement ( 求同法 )

Task 1b:

You may get the answer “vegetable salad” from the Method of Agreement. But the pathology tests showed that there were no bacteria causing diarrhea found in the vegetable salad. The actual cause why these four students suffered from diarrhea was that the forks they used to eat the vegetable salad were contaminated with bacteria. People will be blinded by apparent causal relationship but in fact there are real reasons other than that of observed. For instance, in the malaria case above people believed that the cause of malaria was marsh but the true reasons are that the parasites that cause malaria can be carried by mosquitoes that breed in water.

Activity 2

Micheal Faraday’s Electro-magneticInduction Experiment

Activity 2

Activity 2

1. Assemble the above experimental set-up.

2. Place a bar magnet into the copper coil (no movement of the magnet) and observe the reading of the ammeter.

3. Place a bar magnet into the copper coil and move the bar magnet forward and backward. Observe the reading of the ammeter.

4. Repeat steps (3) and (4) by replacing the bar magnet with an iron bar.

Task 2:

1. Design a table to record the results of the above experiment.2. Suggest the cause of the change (if any) in Faraday’s Experiment.3. Use a comparison table / diagram to show the logic of the discovery by Faraday. 4. Do you know the name of the method (in (3)) of establishing the causal relationship?5. What is the precaution of applying the method (in (4)) in causal reasoning?

Task 2

1. Design a table to record the results of the above experiment.Answer:

Movement of magnet Movement of the pointer of ammeter

Yes YesNo No

Movement of iron Movement of the pointer of ammeter

Yes NoNo No

Task 2

2. Suggest the cause of change (if any) in Faraday’s Experiment.

Answer: It’s the movement of the magnet that

causes the pointer of the ammeter moves.

Conclusion: change of magnetic field causes generation of electricity.

Task 2

3. Use a comparison table / diagram to show the logic of the discovery by Faraday.

Answer:Case/event Circumstance Effect

1 A, B, C X2 - B, C X

Task 2

4. Do you know the name of the method (in(3)) of establishing the casual relationship?Answer: Mill’s Method of Difference ( 求異法 )Remarks: Mill’s Method of Difference includes Experimental and Control groups.

Task 2

5. What is the precaution of applying the method (in(4) in the causal reasoning?Answer:If there are more than one variable in the experimental design, the results obtained are not conclusive.

Activity 3

Discovery of a ‘strange’ disease by Dr. Sun Simiao( 孫思邈 )

Activity 3

Dr Sun Simiao( 孫思邈 ) (AD 581-682) was a very famous doctor in Tang Dynasty. There were several rich people living in the capital of China, Changon. All of them got a strange disease at the same time. Their symptoms included leg edema, muscle pain, fatigue and tiredness. All the famous doctors could not treat the diseases. Dr. Sun was invited to treat one of the patients. He went to the kitchen of one of the rich patients. The cook told him that his master did not like to eat too much meat and fish but he liked to eat polished rice (white rice which had been polished several times). Dr. Sun also paid visits to several rich patients who suffered from the same diseases. He found that these patients had the same habit of eating polished rice.

Activity 3

1. Use a table / diagram to show the logic of the discovery of the cause of the disease.Answer:

Case Conditions Symptom

w x Y (ate polished rice)

Leg edema, muscle pain, fatigue and tiredness

Patient A(rich) (+ve gp)

Yes No Yes Yes

Patient B(rich) (+ve gp)

No No Yes Yes

C(poor) (-ve gp)

Yes No No No

D(poor) (-ve gp)

No Yes No No

Task 3:

2. Do you know the name of the method (in (1)) of establishing the causal relationship?Answer:Mill’s Joint Method of Agreement and Difference( 求同求異並用法 )

Task 3:

3. What is the precaution of applying the method (in (2)) in causal reasoning?Answer: We should consider and compare as many conditions and symptoms as possible among the +ve and –ve groups in order to minimise the chance factor.

Activity 4

The Discovery of Radium by Marie Curie.

Activity 4

In Marie Curie’s time a radioactive substance, uranium was discovered. When uranium compound was placed onto a photographic plate covered with black paper, it produced an impression analogous to that which light would make on that plate. The impression is due to uranium rays that traverse the paper. These same rays are like X-rays, which can discharge an electroscope by making the air surrounds it behaves like a conductor.

Activity 4

One day when Marie Curie and his husband were doing research on some samples containing uranium, they discovered that the radioactivity measured was far greater than that of uranium. They began to suspect that the sample contained other radioactive active substances. Then they carried out numerous experiments to search for the unknown radioactive substance. Finally, after 45 months’ hard work the couple isolated a very minute quantity of black power, radium chloride in 1902.

Task 4

1.Show the logic (method) in the cause of the discovery of radium by Marie Curie.2. Do you know the name of the method (in (1)) of establishing causal relationship?3. What is the precaution of applying the method (in (2)) in causal reasoning?

Task 4

1. Show the logic (method) in the cause of the discovery of radium by Marie Curie.Answer: Symbolically, the Method of residues can be represented as: A B C occur together with x y z B is known to be the cause of y C is known to be the cause of z Therefore A is the cause of xe.g.1. Let A be Uranium and “a” be the radioactivity measured due to the effect of A2. Let B be the unknown radioactive substance and “b” be the radioactivity measured due to the effect of B. 3. Since the radioactivity measured was higher than expected (a + b>a), then the source of the radioactivity must come from radioactive substance (s) other than A.

Task 4

2. Do you know the name of the method (in (1)) of establishing the causal relationship?Answer:The induction method is called Method of Residues.

Task 4

3. What is the precaution of applying the method (in (2)) in causal reasoning?Answer: Only when it is clear that certain part of the phenomenon or observation is due to some recognized reason/factors, can the factor/reason that can account for the remaining part of the observation/phenomenon be confirmed. Sometimes it is difficult to use the Method of Residue to find the causal relationship because the remaining factor found is a compound factor. As in the discovery of radium, Marie Curie first isolated Po (plutonium) and then found radium which contributed to the major difference in the observed radioactivity of the uranium sample

Activity 5

Air pollution increases hospital admissions for patients with chronic obstructive pulmonary disease in Hong Kong

Activity 5

In Hong Kong, Chronic Obstructive Pulmonary Disease (COPD) was the 5th leading cause of death, and accounted for at least 4% of all public hospital acute admissions in 2003. The prevalence of COPD among elderly Chinese (age ≥70 years) living in Hong Kong is estimated to be 9%. Previous studies have shown that pulmonary function and quality of life among patients with COPD were adversely affected by frequent exacerbations.

Activity 5

A study has just been published in the journal THORAX was carried out by the Department of Medicine and Therapeutics and the Department of Community and Family Medicine, the Chinese University of Hong Kong. This study assessed the relationship between the levels of ambient air pollutants and the hospitalisation rate due to COPD in Hong Kong.

Activity 5

Data of daily emergency hospital admissions to 15 major hospitals in Hong Kong for COPD and indices of air pollutants (sulphur dioxide [SO2], nitrogen dioxides [NO2], ozone [O3], particulates with an aerodynamic diameter of less than 10μm [PM10] and 2.5μm [PM2.5]) and meteorological variables from January 2000 to December 2004 were obtained from several government departments.

Activity 5

Significant associations were found between hospital admissions for COPD with all 5 air pollutants. For every 10μg/m3 increase in SO2, NO2, O3, PM10 and PM2.5, there was 0.7%, 2.6%, 3.4%, 2.4% and 3.1% increases in the rates of COPD hospitalisations respectively. O3 had the strongest effect on COPD hospitalizations. The effect of SO2, NO2, and O3 had a stronger effect on COPD admissions in the cold season (December to March) than during the warm season.

Activity 5

Adverse effects of ambient concentrations of air pollutants on hospitalisation rates for COPD are evident, especially during the winter season in Hong Kong. Measures to improve air quality are urgently needed.

(Press Release of CUHK 6 March 2007)

Task 5

1. Use one or two sentences to describe the findings of the Chinese University of Hong Kong?

2. Please use a table/diagram to represent the logic in establishing the causal relationship. Do you know the name of the method (in (2)) of establishing such causal relationship?

3. What is the precaution of applying the method (in (2)) in causal reasoning?

Task 5

1. Use one or two sentences to describe the findings of the Chinese University of Hong Kong?Answer: Air Pollution Increases Hospital Admissions for Patients with Chronic Obstructive Pulmonary Disease in Hong Kong.

Task 5

Evidence: There is a direct correlation between the degree to which the cause occurred and the degree to which the effect occurred.Conclusion: A and x has a causal relationship. A is the cause of x.

Case/event Circumstance Effect

1 A, B, C X2 A+, D, E X+3 A-, F, G X-

Task 5

Concomitant Variation

Task 5

a) Two events have correlation but it is not necessary for them to have causal relationship. For instance, many overweight people would die at young ages. However, overweight is not a direct cause of early death.

b) Many irrelevant factors would increase or decrease concomitantly. For instance, both of the average rain falls per year and the birth rates in Hong Kong have been decreasing. It is not likely that the amount of rainfall affects the birth rate.

c) There is a limit to degree of change in circumstance and effect

d) The circumstance must be the only cause for the effect

Make observations

Analyse and interpret the observations

Draw conclusions about the causal relationships

Mill’s methods inCausal reasoning

Method of Agreement求同法Method of Difference求異法

Joint method of Agreement and Difference求同求異並用法

Method of ConcomitantVariation共變法 Method of Residues剩餘法

What are your comments on Mill s Methods of ‛causal reasoning?

Mill’s methods of causal reasoning

Discovery or Proof ?

- can’t be used to discover the cause of some observed event

- can only be used to establish the correlation between the occurrences of distinct events

Mill’s methods of causal reasoning

Confirmation- as tools for confirming hypotheses - useful if we have already proposed several specific hypotheses about what may be the cause of an observed event