<prompt> Translate the following text to EnglishSehemu fulani ya mabadiliko katika thamani ya soko ya mali zilizopo inaweza kutafakari uwezo wa uzalishaji ulioongezeka na hivyo inaweza kuwakilisha mapato na akiba, lakini ni vigumu kutenganisha sehemu hiyo.  <response>: Some portion of the change in the market value of existing assets may reflect increased productive capacity and thus could represent income and saving, but it is difficult to isolate that portion. 