decline in sth
prep. phr..prep. phr.. a reduction in the amount, quality, or importance of something.
prep. phr.. a noun phrase indicating a downward trend or quantitative reduction in a specific variable, typically used in formal or academic contexts.
There is a sharp decline in sales this month.
The city has seen a steady decline in air pollution since the new traffic laws began.
Economists are concerned that a continued decline in consumer spending could trigger a prolonged recession across the manufacturing sector.
the preposition 'in' identifies the specific area or category that is decreasing.
contrast 'decline in' (the area of change) with 'decline of' (the entity that is weakening); for example, 'a decline in standards' vs 'the decline of the Roman Empire'.
a decline of the populationa decline in the populationuse 'in' when referring to the specific measurement or quantity that is decreasing.