@Book{OL4909786M,
    title: {Piaget's theory of intellectual development},
    author: {Herbert Ginsburg and Sylvia Opper},
    publisher: {Prentice-Hall},
    year: {1969},
    place: {Englewood Cliffs, N.J},
}

@Author{OL383810A,
    name: {Herbert Ginsburg},
}
@Author{OL427645A,
    name: {Sylvia Opper},
}