In thinking about or looking for a causal relationship researchers have to decide if they are dealing with a necessary condition. A necessary condition is one that must be satisfied for the statement to be true. A sufficient condition is one that, if satisfied, guarantees the statement will be true.
Some conditions can be both necessary condition and sufficient condition. A necessary condition is that which must be present for the effect to occur. To put it another way, if B appears then A must have been present. Social sciences seldom deal with necessary conditions, rather they are happy to find sufficient conditions.
Some examples of necessary condition:
A Necessary Condition for the Existence of SW-Monopoles.
Is autonomic dysfunction a necessary condition for chronic peptic ulcer formation?
Quantization as a Necessary Condition for Gauge Invariance.
Risk Aversion a Necessary Condition for Limiting Global Environmental Risks?
Is wage compression a necessary condition for firm-financed general training?