Not all evidence is equal. A central skill in Extended Investigation is the ability to weigh evidence according to its reliability, validity and relevance. This applies both when evaluating sources and when choosing what data to gather for your own investigation.
Ask four questions about any piece of evidence:
| Type | Definition | Strength | Weakness |
|---|---|---|---|
| Primary | Original, first-hand data (your own research, original studies) | Direct, unmediated | Time-consuming to collect; scope limited |
| Secondary | Summaries, analyses or reviews of primary sources | Broad coverage; expert synthesis | May distort or selectively represent primary source |
Numerical data — statistics, measurements, survey results, experimental outcomes.
Strengths:
- Can be replicated and verified
- Allows comparison across studies (meta-analysis)
- Reduces subjectivity in reporting
Weaknesses:
- Can hide nuance and context
- Susceptible to statistical manipulation (“lies, damned lies, and statistics”)
- Correlation studies cannot establish causation
- Sampling errors can invalidate findings
KEY TAKEAWAY: Strong quantitative evidence comes from large, representative samples using valid instruments, with results that have been independently replicated. A single small study is weak evidence, regardless of its statistical significance.
Non-numerical data — interviews, case studies, observations, document analysis.
Strengths:
- Rich in detail and context
- Captures experiences and meanings numbers cannot
- Generates hypotheses for further quantitative testing
Weaknesses:
- Harder to generalise to wider populations
- More susceptible to researcher bias in interpretation
- Less straightforward to replicate
- Can be influenced by participant performance (saying what they think you want to hear)
Strengths:
- Expert knowledge and experience can fill gaps in empirical data
- Useful when empirical studies are impossible or scarce
Weaknesses:
- Experts can disagree — consensus matters more than individual opinion
- Appeals to authority can be fallacious if the expert is not relevant to the question
- Experts may have conflicts of interest
EXAM TIP: If asked to evaluate a piece of evidence, structure your response as: (1) name the type of evidence, (2) state its general strength, (3) identify the specific weakness in this case, (4) explain how this affects the argument.
Personal stories and individual examples.
Strengths:
- Illustrative and memorable
- Can represent genuine lived experience
Weaknesses:
- Not generalisable — one case cannot establish a pattern
- Highly susceptible to selection bias and memory distortion
- Often emotionally compelling but logically weak
Strengths:
- Synthesise findings from many studies, greatly increasing statistical power
- Highest level of evidence in most research hierarchies
Weaknesses:
- Quality depends on quality of included studies
- Publication bias (negative results often not published) can skew findings
When selecting data collection methods, consider:
- Which type of evidence best answers your specific research question?
- What are the known limitations of that evidence type for your context?
- How will you acknowledge these limitations in your written report?
COMMON MISTAKE: Treating all peer-reviewed sources as equally strong. Peer review is a quality threshold, not a quality guarantee. A peer-reviewed study with a sample of 12 is still weak evidence — evaluate the evidence itself, not just its publication venue.
VCAA FOCUS: In your written report’s evaluation section, you must explicitly acknowledge the limitations of your evidence. A report that only claims strong evidence will score lower than one that honestly identifies both strengths and weaknesses.
