Many low income families have irregular income that is sometimes not reported. Some studies try to take these problems into account. For example, most studies count the face value of food stamps as income. But other studies do not. The size of the income effect depends on the accuracy of the income measure.
Most studies look at the effect of only one year of income.
This tends to make the effect of an income look too small. Short term poverty appears to have little lasting effect on measures of children's well-being. Long term poverty is more harmful. Researchers who estimate the effect of many years of income usually find greater effects than studies that only use one year. The best studies try to both control important parental characteristics and use many years of income.