Table 2.
Examples of measures of association in clinical research
| Measure of Association | Definition | Exposure | Outcome | Effect Estimate | Interpretation |
|---|---|---|---|---|---|
| Relative risk (RR)1 | The ratio of the incidence in the exposed group to the incidence in the unexposed group | Vitamin D intake | Melanoma | RR = 1.31 (95% CI = 0.94 −1.82) | When compared with the lowest quartile of dietary vitamin D intake, participants with the highest quartile of intake had 1.31 times the risk of melanoma. This may also be phrased as having a 31% increase in melanoma risk. Because the 95% CI includes 1 (the null value, indicating no association between exposure and outcome), the results are not statistically significant (Asgari et al., 2009b). |
| Odds ratio (OR) | The ratio of the exposure odds among the case group to the exposure odds among the control group | Presence or absence of HPV | Squamous cell carcinoma | Any HPV species: OR = 0.9 (95% CI = 0.4–1.8) HPV β- papillomavirus: OR = 4.0 (95% CI = 1.3–12.0) | This study compared tissue from patients with squamous cell carcinoma to tissue from control individuals with no history of skin cancer. No statistically significant association between patients (cases) and control individuals was observed when all HPV species were considered as the exposure. In the subgroup analysis, however, tissue from patients was 4 times more likely to contain the β-papillomavirus species compared with tissue from control individuals (Asgari et al., 2008). |
| Hazard ratio (HR) | The ratio of the rate at which patients with a risk factor experience an event to the rate at which patients without the risk factor experience an event | Systemic immune suppression | Merkel cell carcinoma-specific survival | HR = 3.8 (95% CI = 2.2–6.4) | The rate of death from Merkel cell carcinoma for people with systemic immune suppression was 3.8 times higher than for nonimmunosuppressed individuals (Paulson et al., 2013). |
| Pearson correlation coefficient (r) | Measures the strength and direction of the linear association between two continuous variables | GOLPH3L gene expression | HORMAD1 gene expression | r = 0.991 | There is a strong, positive linear relationship between GOLPH3L and HORMAD1 gene expression, indicating that when one gene is expressed, the other is often expressed as well (Ioannidis et al., 2018). |
| Spearman correlation coefficient (rho) | Measures the monotonic relationship between two variables | Individual typology angle | Melanin index | ρ = −0.98 | There is a strong, negative monotonic relationship between individual typology angle and melanin index, indicating that when one is low, the other is high (Wilkes etal., 2015). |
| Beta coefficient (linear regression) | Measures the association between a continuous outcome variable and continuous and/or categorical predictor variable(s) | Pain (self-rated from 0–10) | Sleep quality score (range = 8–40, with higher scores indicating more disturbed sleep) | β = 0.21 P < .001 | There is a positive relationship between self-rated pain and sleep disturbance. For each 1-unit increase in self-rated pain, sleep quality score increases by 0.21. The P-value indicates that this association is statistically significant (Milette etal., 2013). |
| Chi-squared Test | Measures the association between two categorical variables by assessing whether there is a significant difference between observed and expected data | Training level of clinician | Treatment type | P < 0.0001 | Patients treated with Mohs surgery were almost exclusively treated by attending physicians (98.8% vs. 1.2% resident/nurse practitioner). Patients receiving excision were treated slightly more frequently by resident physicians (51% vs. 46.8% attending and 2.1% nurse practitioner). Patients treated with destruction by electrodissection and curettage were more commonly treated by attending physicians (57.1% vs. 33.8% resident and 9.1% nurse practitioner). The P-value from the chi-squared test indicates that these differences are statistically significant (Asgari et al., 2009a). |
| Risk difference (RD) | Measures the difference in risk between exposed and unexposed groups | UV light therapy | Psoriasis | RD = −0.06 | After receiving UV light therapy, 2% of patients continued to experience psoriasis, compared with 8% of patients not receiving this treatment. The RD indicates that patients who received light therapy had 6 fewer cases of persistent psoriasis per 100 people compared with patients not receiving light therapy.2 |
| Relative risk reduction (RRR) | The proportion of risk reduction attributable to the exposure/intervention | UV light therapy | Psoriasis | RRR = 0.75 | Using the data from the UV light/psoriasis example, the relative risk may be calculated as 0.02/0.08 = 0.25 (the incidence in the exposed group divided by the incidence in the unexposed group). The RRR is therefore 0.75(1 – RR), which can be interpreted as UV light therapy resulting in a 75% reduction in psoriasis incidence, relative to patients who did not receive light therapy.2 |
| Number needed to treat (NNT) | The number of patients who must be treated for one patient to benefit | UV light therapy | Psoriasis | NNT = 16.7 | Using the data from the UV light/psoriasis example, the NNT may be calculated as 1/ (incidence among the unexposed – incidence among the exposed), or 1/(0.08 – 0.02). Therefore, the NNT equals 16.7, indicating that 17 patients need to be treated with UV light therapy for one patient to benefit.2 |
Abbreviations: CI, confidence interval; HPV, human papillomavirus.
1
Relative risk may also be referred to as the risk ratio, rate ratio, or relative rate.
2
Mock data are used for these examples.