Gauging a stamp's scarcity can sometimes rely partly on a hunch based upon one's knowledge and experience of the stamp market. A collector or dealer who has traded in stamps for decades may develop a "feeling" for which stamps are common and which are scarce, when no solid quantitative information is available.
Collecting stamps on a serious level is a leisure activity which requires that the individual who engages in it have sufficient time and money to do so. Therefore, the number of serious collectors within a country usually correlates with the size of its middle class. The growth of a country's middle class is dependent upon two variables: economic growth, and the extent to which that growth benefits the population as a whole. A situation in which a country's prosperity is not sufficiently shared but mainly benefits only a tiny ruling elite does not augur well for the country's stamp market.
Significant demand for a particular country's stamps may extend beyond the collectors within the country, however. If there exist large numbers of immigrants from the country (or their descendants) living in other countries, such groups may have a cultural or sentimental bond with their former country. Also, as some countries are former colonies of European nations, there may be a demand for a country's stamps among those who collect the former mother country and its colonies. Thirdly, certain groupings of countries (such as the Baltic States, Latin America, etc.) are often collected as regions. Thus, in many cases applying criteria of economic growth may be complicated by the dual or multiple "overseas" appeal of a particular country's stamps.
A number of miscellaneous factors can also significantly affect demand. If a stamp has been extensively forged or faked, but is not valuable enough to be worth expertising, then demand for the stamp may be adversely affected, even if authentic examples are scarce. The reason for this is that most collectors will tend to value even the authentic stamps as if they were fakes, because they will not be able to distinguish between what is authentic and what is not. This is the case with many overprinted issues. If major catalogs consider an issue "illegitimate" (i.e. issued mainly as a means of profiting from sales to collectors, rather than for postal use), and do not list it, then that also may dampen demand. Every once in a while, a postal service will issue a stamp with innately self-destructive qualities, such as high-acid paper or gum, or gum containing chemicals which otherwise degrade the stamp over time. Such stamps present special preservation problems which may adversely affect demand, although since these problems often result in accelerated diminution of supply, collectors who take the necessary steps to preserve such stamps may be amply rewarded.
In summary, the following criteria should be considered when evaluating an issue as an investment:
1) Quantity issued, or if not known, an estimate thereof
2) Estimated quantity remaining
3) The population and economic prospects of the country of issuance (i.e. economic growth and growth of a middle class)
4) "Overseas" appeal of the country's stamps
5) Topical appeal, and trends affecting growth of the topic
6) Miscellaneous factors