The definition of these two terms can be derived from their roots: quali-, as in quality (sensations, experience, ephemeral or perceptual characteristics) and quanti-, as in quantity (things that can be measured, counted, or precisely defined). For example, “That film was grim (qualitative) and was 2 hours long (quantitative).” If you say “…and it was too long” it becomes qualitative. Another way of thinking about this is through the more common notions of ‘objective’ and ‘subjective’. Objective (quantitative) data or information can be definitively defined and defended – “The table is one metre long” can be proven true or false with a tape measure. Subjective (qualitative) data or information cannot be generally proven. The statement “The table is too small” depends on what it is used for and perhaps by whom (a child versus an adult).
Design criteria, research data and site information can be broken down into either of these categories and recording them requires different techniques. For example, qualitative information or data may be recorded through interviews, observation, sketching, photograph, sound recordings, mental maps, and so on. Quantitative data may be gathered by measuring, counting, weighing, documentary photography, measured drawings, and so on. The way you present these in your research or portfolio will therefore differ.
Understanding this difference can help you unravel a design brief. You also avoid making unnecessary assumptions about your site, program or project research. The architect has the difficult responsibility of understanding and satisfying the needs, not just of the client but of the many subsequent users and non-users (passers-by) of architecture. The perceptual point of view of the architect is but one subjective view among many others. The architect’s subjectivity, as such, is no more valuable than anyone else’s. Your task is not to privilege your point of view over any others but to develop skills that allow you to work with viewpoints, interpretations, opinions and other subjective criteria in a measured, appropriate and productive manner. Further, you must weight and balance the relative importance of objective and subjective criteria and information according to the demands of the particular brief. Understanding the difference between these two terms is an important part of this.