Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

If throwing good money after bad is generally a bad idea, is throwing more money after good Ok?
I have heard that investing more money into an investment which has gone down is generally a bad idea*. "Throwing good money after bad" so to speak. This is over simplified statement to explain the concept. What is essentially says is; Say I hold stocks of XYZ; 100 units worth say USD 1000. This has lost me x% [say 50%]. The general tendency is to buy 100 more units in anticipation / hope that the price will go up. This is incorrect.  However on case to case basis, this maybe the right decisions. On a periodic basis [or whenever you want to invest more money]; say you have USD 1000 and did not have the stock of XYZ, will you buy this at current price and outlook of the company. If the answer is Yes, hold the stock [or buy more], if the answer is no sell the stock at current market price and take the loss. The same applies when the price has appreciated. If you have USD 1000; given the current price and future outlook, will you buy the specific stock. If yes, hold the stock [or buy more], if answer is no sell the stock and book profit. Off-course I have not overlaid the various other considerations when buying stocks like diversification, risk profiles of individual stocks / segments, tax implications etc that are also essential even if you decide to buy or sell specific stock.