$ ext{Marginal propensity to save ($\Delta$ = change, y = Income \& s = saving) =} $

Q: $\text{Marginal propensity to save ($\Delta$ = change, y = Income \& s = saving) =} $

$\text{Marginal propensity to save ($\Delta$ = change, y = Income \& s = saving) =} $

Question

$	ext{Marginal propensity to save ($\Delta$ = change, y = Income \& s = saving) =} $

Accepted Answer

The marginal propensity to save (MPS) is a concept in economics that measures the proportion of additional income that is saved rather than spent on consumption. It is an important component of Keynesian economics and helps in understanding consumer behavior in response to changes in income.

1. **Explanation for Option 3 ($\Delta s / \Delta y$):**

Option 3 is the correct answer because the marginal propensity to save is defined as the ratio of the change in saving ($\Delta s$) to the change in income ($\Delta y$). Mathematically, it is expressed as:
   $$
   \text{MPS} = \frac{\Delta s}{\Delta y}
   $$
   This formula indicates how much of an additional unit of income is saved. For example, if an individual's income increases by $100 and their savings increase by $20, the MPS would be 0.2, meaning 20% of the additional income is saved.

2. **Explanation for why other options are incorrect:**

- **Option 1 ($\Delta y / \Delta s$):**
     This option suggests the inverse of the correct formula. It implies the change in income per unit change in saving, which does not align with the definition of MPS. MPS specifically measures savings behavior in response to income changes, not the other way around.

- **Option 2 ($y / s$):**
     This option represents the ratio of total income to total savings, which is not relevant to the concept of marginal changes. MPS focuses on changes in income and savings, not their absolute levels.

- **Option 4 ($s / y$):**
     This option indicates the average propensity to save, which is the ratio of total savings to total income. While related, it does not capture the incremental changes that MPS is concerned with. MPS is about the additional savings from additional income, not the overall savings rate.

In conclusion, Option 3 ($\Delta s / \Delta y$) correctly captures the essence of the marginal propensity to save by focusing on the incremental changes in savings relative to changes in income, which is why it is the correct answer.