Essentials of research methods in neurosurgery and allied sciences for research, appraisal and application of scientific information to patient care (Part I)

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Figure 1
Types of common research designs in neurosurgery.
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Figure 2
Case report(s) and case series.6
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Figure 3
Anatomy of case series in neurosurgery.6
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Figure 4
Anatomy of the descriptive cohort design
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Figure 5
Anatomy of a case-control study.
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Figure 6
Anatomy of a cohort study (prospective)

Table 1

The classic contingency.

Exposure	Outcome (disease=D⁺) for example Tumor recurrence	No outcome (no-disease=D^-) for example No tumor recurrence	Total
Intervention (exposed=E⁺) for example Dr. JJ’s new approach	Exposed with outcome =a (recurrent cases operated via Dr. JJ’s approach)	Exposed without outcome=b (non-recurrent cases operated via Dr JJ’s approach)	All exposed persons =a+b (all patients operated via Dr. JJ’s approach)
No intervention (non-exposed=E^-) for example sub-frontal approach	Not exposed with outcome =c (recurrent cases operated via subfrontal approach)	Not exposed without outcome=d (non-recurrent cases operated via subfrontal approach)	All non-exposed persons =c+d (All patients operated via the subfrontal approach)
	All persons with outcome =a+c (all patients with tumor recurrence)	All persons without outcome =b+d (All patients without tumor recurrence)	All persons a+b+c+d

Risk or incidence of recurrence= a/a+b (for Dr. JJ’s approach) & c/c+d (for subfrontal approach), relative risk (RR)=[Risk in exposed]/ [Risk in the non-exposed]=[a/a+b]/ [c/c+d], odds of exposure in the cases=[a/c] and odds of exposure in the controls=[b/d], odds ratio (OR)=[odds of exposure in the cases]/[odds of exposure in the controls]=[a/c]/[b/d]

Table 2

Summary characteristics of cohort, case-control, and cross sectional studies.25

Characteristics	Cohort study	Case control study	Cross sectional study
Population at risk	Defined at the beginning	May be undefined	Begins with a defined population
Cases	Not selected but ascertained by surveillance	Selected by investigator from an available pool of patients	Not selected but ascertained by a single examination of the population
Controls	Not selected, evolve naturally	Selected by investigator to resemble cases	Include those free from disease at the single examination of the population
Exposure	Measured before development of disease	Measured, reconstructed, or recollected after development of disease	Measured at same time as disease
R or I and RR	Measured directly	R and “I” cannot be measured directly; RR of exposure estimated as OR	R and I cannot be measured directly; RR of exposure estimated as OR