Let me say before I start that my information and opinions come from my educational and experiential background - I am a special education teacher, have a BA in Psychology, an MA in Counseling, and am working on certification as a school psychologist.
IQ tests, historically, were developed for several reasons, primarily to determine which students should be sent to secondary school (that is, past 6th grade). Another use to which IQ tests were historically put was to determine (back when there were plenty of volunteers) was to screen applicants to the armed forces so that the best could be chosen from those who volunteered. A third frequent use was to screen potential immigrants to the US. These latter two uses were prejudicial - especially since early IQ tests leaned heavily on acquired knowledge, which could only occur through formal education (or, in rare cases, extensive self-education) and the ability to manipulate the English language - therefore, people who did not have access to education or books, and who were not native speakers of English, were placed at a severe disadvantage. In addition, much of the information assessed had a social context that was not necessarily the same as that of the applicant - most tests were normed (standardized) to a middle class or higher socioeconomic stratum, and those from lower strata were at a disadvantage. This is still the case when assessing students from other countries and from minority populations (by parental educational level, by economic status, by cultural group, etc.).
While certain IQ tests now have tasks that assess ability based on non-verbal tasks (completing puzzles, copying non-alphanumeric symbols, etc.) most rely on the student having a well-enough developed English language capacity to understand and correctly follow the assessor's directions. For students who are not from the cultural/economic mainstream of the society in which the assessment norms are determined, they are at a disadvantage both because they are less-likely to be familiar with "appropriate" developmental tasks and abilities, and because their cultural norms are generally not included in the assessment manual, which makes interpreting their results problematic. Even for those assessments available in this country in other languages, the assessment norms are based on socioeconomic norms which may not apply to the student - one of the most popular, for example, is available in Spanish, but was normed on American students of Hispanic descent who grew up in Spanish-speaking homes in America - much different from Spanish-speaking students who come from Mexico City, and again from those from rural Mexico, again from those from Brazil, again from those from Spain, and so on.
In addition to these concerns, two people who have identical IQ scores can have widely different abilities based on personal experience, educational and experiential background, gender, and numerous other factors - both may have strength in hands-on activities, for example, but one may have demonstrate that facility through carpentry, and another may demonstrate it through art, even though both look the same in terms of their achieved scores on IQ tests.
Because of the many factors that can affect IQ scores, they should be taken with a very large grain of salt - while IQ scores are highly correlated with success in school, they are not as correlated with success in life; certainly, people who do well in school are more likely to do well in life, but other factors can affect that as well - motivation being a key factor. A highly motivated person with an average IQ may excel in both school and life, while a minimally motivated person with an above average or even gifted IQ may drift through school and then life.
In addition, there are many types of intelligence that are not assessed by commonly used intelligence tests; there are also many people who have the intelligence but don't test well.
Having an IQ that tests high is not a guarantee of success; likewise, having an IQ that tests low is not a guarantee of failure. Human beings are complex, and no one descriptor can be taken as the answer to what causes success and failure; rather, a person must be considered as a sum of his/her experiences, abilities, motivations, and desires, and not shunted into a slot because of a number on an assessment which is culturally, linguistically, and economically biased, and which contains sections for which the interpretation is rather subjective.
Remember, too, that the only measure of success that means anything is your personal one. I have a TKD student who is 43 years old and has Downs Syndrome. He is, in his own opinion, very successful - he has held the same job for over 10 years (grocery sacker), lives independently, has earned a yellow belt in TKD, and has a wide range of friends - and for his achievements, he has earned the respect of everyone who knows him. His IQ is, relative to the norm of the culture, low - but his motivation has always been high, and he is, in his own opinion, the most successful person he knows, and one of the most content adults I have ever met.
