Can you compute (i) the own-price elasticity of demand for Times, (ii) relevant cross-price elasticities of demand for Guardian, Telegraph and Independent? How do you ensure that the ceteris paribus assumption has been met here, although approximately?

Can you compute (i) the own-price elasticity of demand for Times, (ii) relevant cross-price elasticities of demand for Guardian, Telegraph and Independent? How do you ensure that the ceteris paribus assumption has been met here, although approximately?