(a) Following are the attributes that a population has but an individual organism does not have. 

(i) Birth rate: Per capita births

(ii) Death rate : Per capita deaths. 

(iii) Sex ratio : Ratio of number of males to females in a population 

(b) Population density : It means number of individuals present per unit area. Population density can be measured by determining the population size. 

The different methods to study population size are as follows : 

(i) Quadrat method : It is a method that involves the use of square of particular dimensions to measure the number of organisms. e.g., The number of parthenium plants in a given area can be measured using the quadrat method. 

(ii) Direct observation : It involves the counting of organisms in a given area, e.g., In order to determine the number of bacteria growing in a petridish, their colonies are counted. 

(iii) Indirect method : In this method, there is number need to count the organisms individually, e.g., number of fishes caught per trap gives the measure of their total density in a given water body.